As someone who's fought off DM burnout from over-prep but does a semi-sandbox style, I recommend that you buy a lot of modules, especially those with starter towns, then sprinkle them over the map. Keep a log of adventure hooks. Sprinkle references to them periodically through the game.
First, there are quite a few places in the Nentir Vale that can be expanded for adventuring (Temple of the Yellow Skull, Raven's Roost, the tombs beneath Fallcrest, etc.). For each of those I would either a) find an appropriately themed published adventure to stick in at that location and reskin whatever needs to be reskinned; or b) create two or three set-piece encounters for each location with interesting terrain, monster combinations and treasure then 5-6 more "normal" encounters with groups of monsters that make sense for the surroundings (i.e. human bandit minions in Raven's Roost, cultists at the TotYS). You might also want to create a small wandering monster chart for each location and its surrounding area. Come up with a few seeds to plant with the PCs that might make them interested in visiting these places.What are the key things that a lazy DM like me needs to do in order to make player choices matter? I want to do as little work as possible, while still making sure I hit all the bases. What do I need to do, what can I put off until inspiration hits me, and are there any tips you have to help me do this?
Right now I've got a map, various "powers" in each location on it, and some idea of a far-off threat. I have a couple of wandering monster tables, but no lairs or other dungeons as yet.
(I'm using the Nentir Vale in the 4E DMG.