Well depending on the money you have to spend, you could try Cedar Works:
http://www.cedarworks.com/design

Items like swings and slides need a certain clearance, and they could tell you over the phone what would work.

I would get a stand-alone slide, and an inflatable wading pool for use in the summer. My kids (both 3) are really enjoying our climbing dome, which is about 10'X10', and I like that it is both a climbing toy, and they use is as a pretend house/fort (sometimes with an old sheet or picnic blanket draped over it).
http://www.onestepahead.com/catalog/product.jsp?productId=534748&cmSource=Search

You might also think about a small plastic picnic table, then you could keep sand in an under-bed storage box and have a temporary "sand table", and remove it and use the table for playdough, finger-painting, or other messy art stuff, or just to eat lunch on.

We got a small metal one that our little one loves at Toys R Us. Watch for sales in the paper.

Bump. Any other suggestions for a small backyard? Better just to get a playhouse or will they outgrow that too fast?

check out playnlearn (www.playnlearn.com). You might be able to get a play structure that has a swing and a house with a sandbox underneath. My kids are 6&4 and still love their swing set. We used to use a small sand table on our front stoop, but they outgrew it.

Maybe an outdoor fan, and a water/sand table?

if you have a sturdy tree, consider a tire swing from a branch. Then get one of those plastic playhouses and a stand alone slide for your space.

We have a teeny, weeny DC backyard, about 20' by 35'. I'd really like to get a small play set for my kids (3 and 1), but I can't seem to find anything (with a swing and slide) that will fit, assuming the recommended 3-6' space around the equipment. Any suggestions for play sets for small yards? Would we be better off getting a sandbox, playhouse, etc. and forgoing the swings and slide? Of course, this is assuming the mosquitos even allow us to be outside this spring/summer, but that's a topic for another thread.