Carrying a length of spare cordage (eg paracode or similar) is always a good idea. You can used it as a washing line, a food bag hanging line, for hoisting a pack if you want to do some rock scrambling, fixing or extending guy lines, tying up enemies... this list goes on....but having some already tied as ~2 yard prussic loop using two Triple Fishermen's Bends is useful in many ways...I have used it for such things as a sling/bandolier for a SMART water bottle for a day hike (I tied a clove hitch around the neck), suspending a CNOC Vectro when filtering water, extending a guyline when camping on rock and an improvised belt when I forgot to bring one for my hiking pants. The prussic loop's size can be adjusted for light duty things by pulling knots away from each other.
The Clove Hitch is useful to know for temporarily attaching a line to a branch (method 1) or attaching the middle of the line over a post or bottle top (method 2).
The Alpine Butterfly is another very useful knot. It allows you to create a non slip loop in the middle of a piece of cord that can be fairly easily removed. You can quickly fashion a bandolier from the prussic loop by adding a few alpine butterflies to hang some gear in addition to the clove hitch for the water bottle.
The Bowline is another useful knot for putting a removable fixed loop on the end of a piece of cord but often its the middle of the length where you need the loop.