There is a wide range of folding or inflatable kayaks out there. Some are toys and some are serious boats. As you might expect, the good ones are not cheap.
We have a pair of single Feathercraft kayaks and one double. They're great, very seaworthy bluewater boats that can carry a lot of gear. There really isn't a body of water I would hesitate to put them on provided I was physically up to the task. We bought them so they could fly as baggage in large and small planes to places we couldn't transport hard shell boats to.
The good ones typically take an hour or so to set up, and take a bit to break down and pack away, so they're not good for just doing a little paddle on a lake and being on your way. Inflatables can be up and ready to go in 10 minutes.
I would find a way to experiment with several before you buy, especially if you're considering an expensive one.