- How often should you be putting your lens cap back on?
Sometimes I leave my lens cap in the car or even at home. But generally I put it on when I put the camera back in its bag. I don't care much for dust, because there needs to be a lot of dust for it to affect the picture.
I have lens hoods on all my lenses, which protects them from physical damage. I swing them around and into things and the lens hood takes the hits, not the glass inside the lens.
- How can I take (or edit in post) images where there's both a lot of dark and one part that's really bright? Pretty often I'll take a picture of something like a normal/dark-ish lit hallway with one window open to the outside, and it's hard to not make that window completely overexposed.
Besides shooting in raw, there's not much you can do without adding to your kit. With a tripod, you'd be able to take two shots (one dark one for the window, and one bright one for the shadows) and combine them in various ways. But good tripods cost a bit of money, probably at least $100 if you get it used. Cheap tripods might not hold your camera still, which is what you want one for in the first place.
- What's the best way to carry the camera when it's in use? The neck strap definitely won't do because I don't want it swinging around like crazy. I'm thinking of maybe one of those belt holsters?
Belt holster would work. Take a look at Black Rapid-style cross-body straps too. I've climbed and run and done all kinds of activities with one of those.
- I don't have a great concept of how small to make the aperture if I'm taking a pic of a huge room and want a large depth of field. I usually just go to ~f/18 and use my tripod, but I know you want to avoid small apertures because of diffraction and I also suspect that's excessive.
The old rule of thumb says that you should not go higher than f/11. I rarely need to go higher than f/11 but of course I would if I needed to.
Generally, your lens is much sharper at f/18 than at f/4, so that probably tells you something. Here's a good read: http://petapixel.com/2013/03/05/overcoming-my-photo-entekaphobia-the-fear-of-shooting-at-f11/