Thanks for the suggestions everybody.
The kid's will definitely be outside as much as possible. However, like most kids, we probably spend more time getting them dressed than they actually spend outside. Sometimes it's just easier to have an indoor option that's as simple as saying "go downstairs and play".
They already have scooters, a hand-me-down drum kit, corn hole, and an old kitchen table to play on. None of that is really great for burning energy, though.
The balance beam is a good idea. My daughter had a birthday party at a local gymnastics gym and really liked the beam. We might be able to fit a swing, too.
We liked the idea of monkey bars and a slide too. That would require more padding than the foam pieces we have now.
Here was a DIY idea for monkey bars in case anyone else needs inspiration:
Monkey Bar Plans