@ALTKey this is a great post. I have a lot of backpacking experience, from various regions in the U.S. (Arizona, the Pacific Northwest, Colorado, the Midwest) and South America (particularly in Patagonia and the Andes), which have caused me to have to deal with a wide range of temperatures and climates.
I want to tell you that a lot of the items you bring will depend a lot on the particular season and climate of your trip, as well as your trip's length, and if you are traveling with anyone else.
If you have a particular location/trip in mind, don't be afraid to reach out if you want more specific packing advice. However, I am going to try to give you some tips that I have learned/items that I bring that others have yet to mention.
First, I want to address the last two questions you posed to @BlueRidge.
1)For cold nights in a hammock, I actually put a lightweight insulated inflatable sleeping pad in the bottom of my hammock. It might seem a little odd, but it's actually helped A LOT. If sleeping in a hammock and it's cold, I'll also either sleep in a lightweight thermal sleeping bag (one that is meant for backpacking)(you can remove the sleeping pad if you use the thermal sleeping bag in the hammock), or I pack a compactable lightweight insulated/thermal blanket + beanie + warm clothes, sometimes even adding a tarp/rain fly of sorts over the top of the hammock to keep away moisture and to trap in your body heat.
2)For sleeping bags, make sure to look for one that's meant for backpacking, as this will be more lightweight than your typical sleeping bag (Unless I am staying somewhere colder than 20 degrees Fahrenheit, I use the REI Co-Op Joule 21 sleeping bag, which is also built for 3 seasons and does great in damp conditions. I believe the men's version of this bag is the REI Co-Op Igneo 17). Regardless of where I am camping, I always go for an inflatable insulated sleeping pad (Personally, I use the REI Co-op Flash 3-Season pad)... I also always bring an inflatable pillow with me (Sea to Summit has awesome ones, and they are available at REI) (I use their Aeros Ultralight pillow). All of these items are light, don't take up a lot of space, and are designed for various climates.
Unfortunately, due to the lighter weight and added durability of backpacking equipment, it tends to be more expensive than their heavier, bulkier counterparts. When buying backpacking gear I try to buy items that are meant for 3-4 seasons, so I don't have to spend extra money to buy special equipment for just a single season (the exception to this rule is when I am in alpine, sub-zero temperatures). However, this is a great way to use your REI dividend, or to wait for these items to go on sale!
Second, I tend to use a lightweight single-person insulated/thermal backpacking tent instead of a hammock, because then I don't have to rely on having trees at certain distances from each other in order to have a place to sleep. On cold nights these tents are great, too, because I can literally spend my entire evening/night inside the tent, keeping me warm (as my body heat remains inside the insulated tent) and unexposed to the elements. ALPS Mountaineering has some amazing tents to choose from. My go-to single-person tent that I use more than anything is their Lunx 1-Person tent. REI Co-Op also has the Half Dome tent series which I love to use.
Third, here are some items that I consider to be essentials no matter where I am backpacking:
-first aid kit that, besides the typical components such as pads and bandaids, includes a blister kit, an advanced wound care kit, alcohol wipes, emergency blanket, triangle bandage, antibacterial ointment, Ibuprofen, Tylenol, Aspirin, Benadryl, sugar gel, electrolyte tablets, athletic tape, trauma shears, disposable gloves, a couple of ziploc bags (to dispose of contaminated items such as gloves, if need be), and an ACE bandage (I might also recommend throwing in a mask if you backpack during the COVID-19 pandemic). NOLS has some great pre-made first-aid kits.
-toilet paper, duh, a large ziploc for your TP, and a female disposable period pad (even if you aren't female, this can be handy if you have an injury that requires an absorbent pad... it might seem kind of odd, but it's actually helped me in the past)
-plastic rain poncho - this is a great, tiny tool that helps if you get stuck in a downpour, but can also double as a first aid supply or plastic if you need to seal anything
-Medical History Information Card - Keep this card on your person (in a pocket, for example), at all times. This is important because if you have a medical problem and are unable to communicate with someone who finds you or who is already with you, this medical history card will provide a lot of useful information, especially once the First Responders or other medically trained individuals arrive. On this card I would typically include age, gender, known allergies, any current medications (even something as simple as Claritin or Ibuprofen) pertinent medical history (chronic illnesses, history of cardiovascular issues, diabetes, history of seizures, etc.), and if you have had any abnormal intakes (abnormal food/liquid consumption) or outputs (diarrhea, vomiting, abnormal pee color, etc.). This is also a good time to note that you should always make sure at least 2 people (that aren't backpacking with you) have a detailed plan of your backpacking plans. I also recommend informing your fellow backpacking buddies of any important medical history (for example, I always make sure my companions know about my chronic left kneecap dislocation issue). Furthermore, I recommend leaving a detailed plan of your trip in the dashboard of your vehicle. If something happens and you go missing, but someone finds your vehicle, that plan can help rescuers locate you.
-SAT (satellite) phone + extra batteries (keep this somewhere water-proof!) - this, again, is important in case you have an issue in the backcountry and can't hike out
-physical trail maps (again, keep these in a ziploc or somewhere else water proof!) if you are on a trail you are unfamiliar with
-electrolyte tablets and back-up water-purification tablets
-talcum powder - all you need is a small amount, and this is a great item to place on your feet to prevent blisters (I apply it in the morning before I hike for the day, and then throughout as I deem fit) and to put in your armpits to reduce some of those funny smells
-extra socks and underwear - nothing feels worse than having to wear wet socks or 3-day-old underwear. Using wet socks can also cause blisters, and using soiled underwear can result in rashes and other nasty things. These are the few items that I tend to pack extras of, for those very reasons.
-superglue (just a small bottle) - you have no idea how useful this is if your backpack or something else suddenly decides to tear while you're 5 days into the backcountry. Plus, this can be used as an emergency medical supply if you have nothing else.
-waterproof bags - I always pack my items in waterproof bags because I am paranoid of getting things wet and having some essential piece of equipment or clothing becoming unusable
-hand sanitizer - especially if you are backpacking somewhere where it's difficult to wash your hands all the time (which is the majority of cases, especially if you're short on water), bringing a small bottle of hand sanitizer is a great way to disinfect your hands post-bathroom session or prior to eating
-waterproof matches/something to light a fire with
-headlamp + extra batteries - last thing you want is to be trying to navigate in the dark without light. I like to use a headlamp because I can go hands-free, and if I'm in my tent there's usually somewhere I can hang it from to provide light. My personal favorite for headlamps is the Petzl Actik Core Headlamp.
-snickers bars!!! Also one of my favorite candies, but a great and quick way to get extra sugar/salt/calories without having to use up water. These are great for backpacking, and are even more valued in emergency scenarios where you run out of food, or don't have enough water to make a dried meal.
Hope this helped! Don't be afraid to reach out with more questions, and enjoy backpacking through nature!
one or more gifts or other benefits from the co-op.