For couples and small child family outings on calm lakes and calm river sections I would consider a 2 person canoe over kayaks. More room to spread out.
For more adventurous explorations (eg rougher water) single kayaks are better assuming you are of similar enthusiasm. You can choose different boats to suit your particular physiques. Tandems a bigger so harder to transport and store and heavier to paddle solo. Could be good in a family setting or if one partner is less enthusiastic but they are a bit more special purpose.
There are various types of kayak depending on your intent. Briefly, Sit ons are good for quick jaunts to the small lake or beach or for a fishing platform in calm waters. Sit ins are best for rougher water and more adventurous expeditions. Inflatables can expand your range while keeping storage and transport easier but there is a range of quality from pool toy to sea capable so know what you are buying.
What safety gear you need depends on where you are going, for how long and when. A PFD and a whistle are the standard items for any boating. For an afternoon by a small lake with other similar users about you may not need much else, beyond the obvious things like appropriate sun protection (water reflects the sun so you need more) hydration and calories (eg a picnic). If you plan longer more remote trips in rougher waters or with motorized traffic you will want to be more fully equipped but you should research what you need specific to that type of trip.