Mine is that age right now and the winter weather is moving in. We get a LOT of mileage out of these ideas:
Step stool up to the bathroom sink, run some water, practice brushing teeth, brushing hair, turning lights on and off ad nauseum with the switches right there.
Step stool up to the kitchen sink with some water, maybe a cup and a few random kitchen tools like ladles and whisks. Bonus if there are 2 sinks to work with!
Especially if it's a cloudy and dim day-turn on a flashlight and let him carry it around. He'll spend 20 min. solid shining it everywhere and experimenting with the light. He loves it! Later on if he needs more interest with this, I'll try shining it through colored cups or playsilks.
A CD of favorite kid's music (ours is Peter Himmelman) playing in the background-for some reason DS loves taking this time to chill alone and work quietly on something like puzzles or car play.
Check with your library to see if they have a story/playgroup time for young ones. Ours has one for ages 1-3 and we just started going. He loves it! We stay afterward so he can play in the kid's corner with some other kids his age. A huge hit. Plus I always come home with new books for us to read later.
Other than that, well, it's always nice to get out of the house, and shopping with one is easier than shopping with two.