Yeah. I prep stuff, but minimally. Like
@EzekielRaiden, it's more ideas, situations, encounters, etc., rather than super detailed prep like you'd get from a module or AP. If I know they are headed for a fey forest, those ideas, situations, etc. will be fey/forest themed. Then the players go where they go and do what they do, and those prepped things either enter play or don't, depending on what happens. And I improvise the places in the middle.
Going back to the fey forest trip, if on the way they look for an inn on the road, there might be one. I don't know. I'd give them a roll or roll behind the screen and if the DC I assign is exceeded, they find one. That inn will be improvised, but I've done soooooooo many inns in my DMing life, and been in soooooooo many inns as a player, it's not hard to come up with a good inn on the fly.
The reason for my light prep and improv mix is that I just don't have the same amount of time to put into prep that I did before I married and had kids.