Hmm. How old do you think of as toddler? Legos might be a little worrisome since you might eat the pieces, but I think we can bend the rules if you like them.
I'd suggest a few rules and as many props as you can get your hands on. By rules, things like making sure you drink a certain amount or certain frequency (a glass of water an hour maybe) and you have to be in diapers of course. Then look at whatever toys you have, movies, games, blankets, any fun or colorful clothes. Put out lots of stuff, and be willing to mix it all up for a night. Maybe your plushies want to play with the hot wheels, or you take everyone flying on a blanket magic carpet, or you can put on a movie and act it out with all your toys as it's going. It's imagination, so the sky is the limit.
And of course, you'll need to have cleanup time at a certain point, and a strict bedtime. Little ones need their rest.
