Here is a project I did for my kids - we live in Alaska where it's either snowing or raining most of the year. Summer lasts about a day and a half (only a minor exaggeration) so I thought the kiddos would appreciate an indoor space to get some exercise. Plus, it gives us additional floor space, which is great.
We have a swing hanging from the ceiling, and an 8' slide (about 4' off the ground at the top), and a ladder. It's like a little indoor playground. We put hooks for dress-up under one half of the loft, and haven't decided what to do in the other half, but we just bought a bean bag at a garage sale and put all the stuffed animals in it. I tossed it in that corner and it's a nice place to read books.
On top was a great place for my daughters dollhouse (made by my father for me when I was little and redecorated by me recently). We used nailed down carpet squares on top to make it cozy. The whole project was under $200, largely because the top is made out of recycled solid core doors we got for free. I'm sure you could use plywood though. I still intend to get some snake lights for underneath lighting. One thing I added since the pictures were taken is a recycled steering wheel on the top middle 4x4. My son LOVES it.
I did all of this while pregnant, so you can do it too! It's *almost* completely freestanding, so you could probably even do it in a rental.