Our basement is only partially finished with open rafters and cement floor so I covered about a third of the floor with foam tiles and put in a (large) Little Tykes slide, a Step 2 Kangaroo Climber, and a Little Tykes playhouse. (mostly for my home daycare kids)
We also hung a swing, a set of rings, a trapeze bar and a knotted rope from the rafters and an industrial cargo (shipping) net on one wall for climbing.
We have a ping pong table (for us and our own kids) a dart board (for us) and a room full of wood ends from Dh's various sites that our own kids use to make things with their hammers and whatever nails they can scrounge up.
I'll bring in the trikes and ride ons when winter hits. Our basement entryway is huge, and it's empty so it makes an awesome room for little guys to drive in.
I have fabric tunnels, a tent, a spring horse and huge foam blocks for building as well, but only take them out once in a while to change things up a bit.
Sometimes, when I can get my hands on a large roll of paper I duct tape a length of it to the wall and the kids can paint a mural (that eats up a couple hours of rainy/snowy afternoon )
No little toys though, just the equipment, like a playground, easy to clean, easy to organize.
This gets me through our looooong spells of crummy weather with my sanity intact.
