If you live in an apartment or have a really small yard, you may feel like your kids can't play outside. There are lots of things they can do though without a lot of space.
Hopscotch and jump rope are oldies but goodies. I bought my kids a five dollar jump rope at Target a couple of weeks ago and they've been going non-stop. They count their jumps and have been keeping a record of who jumps the most times in a row. Jump roping takes virtually no space and can even be done in the house if you're an extra nice mom and you clear a little space in the family room. With a longer rope kids can play together with 2 turning and 1 jumping. Jump rope is also a big calorie burner. I think you'll also find your kids invent a lot of other uses for the jump rope as well.
Hopscotch takes a little more space, but can be done in a driveway, on a sidewalk, or on a back patio. Draw the hopscotch with chalk, or use a stencil to paint a permanent one. When I was a kid I loved my rainbow hoppi-taw, but you can use about anything as a marker. There are forms of hopscotch played all around the world.
If you don't have space for a big trampoline, buy a small exercise trampoline. You can find them at most sporting goods stores. This is an especially good way for pre-schoolers to get their energy out.
Other activities that don't take a lot of room: Pogo stick, tether ball, four square and hula hoop.