A few months ago, on another Joe Forum, they had a post about toys that were compatible with Joes, but weren't from the GI Joe Line. It gave me a lot of ideas (and cost me a small fortune on ebay!!) But some of the stuff they mentioned, I bought and these playsets work well with GI Joe. I'm gonna mention ones I know of/ bought, and if anyone else has playsets that work well, chime in! (Let's forgo the obvious like PTE, BBI and Corps! and try to cover non military figures and sets!)

Aliens Deluxe Playset (from treehouse kids) I used it for my viper pit review!

Star Wars (there are a lot here from the Mos Eisley Bar set to the Endor Bunker and the Ice Planet Hoth set)

MASH 3 3/4 inch vehicles and base (I own two bases and they have a ton of dio accessories!)

Spawn Alley Set

Steel Monsters Line from Tonka (they are post-apocolypitc vehicles ala Mad MAx and go great with Dreaknoks!)

Captain Power Trans-field Communicator and Base set

Batman Wayne Manor (any version)

Ghostbusters Firehouse

Terminator 2 bio-flesh regenerator (it's a little big, but makes a Great background piece for MindBenders Lab!)

Playmobil (anything!)