Having some play equipment in your own back yard is a great thing. When your kids are small and need your supervision while they're outside, you may think it's just as easy to walk them to a nearby park, but as they get older and can play outside without you, they will get LOTS of playtime from their own backyard items.
Don't limit yourself to a traditional playset. Consider a really great sandbox (very cheap to build), a playhouse, a tent, a tire swing. Our girls' trampoline got more hours of playtime than any other single item they had in their upper-elementary years. Sometimes they'd just go outside and lie on it to read. They camped out on it a couple times.
Outdoor play items don't have to be purchased new (in fact, it's wise to purchase them used), but this isn't an area in which to skimp. These things will be enjoyed for years to come and will last for future children.