What if you can't make it to the gym?

I'm starting to realize more and more that with my current schedule I just don't have much time to really go to the gym. At least not as much as I'd like to. And with some upcoming situations, I may have even less time. So, I need to find some other things I can do at home rather than going to the gym. Here are few ideas that I've come up with but if anyone reads this and has some more ideas I'd appreciate them:

Varied Position Pushups
Pullups (probably on the park equipment near my apartment)
Get a set of around 30lb dumbbells, that would allow for a decent workout for arms and shoulders.
Chair Dips
Lunges
Calves on the stairs.

That's all that I can think of right now. I'm just hoping that I can gain the muscle I'm after by doing more of these kinds of workouts, but I'll still need to find a way to work in some good bench work and squats.