Regarding tent camping: - If you want to tent camp you shouldn't worry too much about finding a place. True it's first-come, first-serve, but as long as you're not showing up after mid-afternoon you should be able to find something. If you're getting in late the first day you might want to book a hotel somewhere that night then go to the campground you want to stay at first thing the next morning, find an empty spot or one where people are leaving, set up, pay for your day(s) and go out exploring. Remember it can be in the 30s at night even during the summer, so make sure you're ready for that type of camping. Also, with the size of the park it may make sense to camp for a few days on one side, then move for a few days to another area (i.e. two days at Madison campground then two days at Canyon campground).

When it comes to hotels it's just like Disney, people are always canceling/changing/making plans so something that's booked this morning may be open tomorrow. The prime hotels fill up just about as soon as the booking window opens.

The place is huge and there's so much to see and do that a couple days visit is going to be like trying to see all of Disney World in one day - you can probably just barely see all the highlights in that time but it's better than never seeing it. The Old Faithful geyser area is part of a huge geyser basin and there are two other basins nearby so just seeing the old faithful area can be a full day in itself.

If you're up for some hiking by all means do it, get away from the roads and the touristy spots. Seeing Old Faithful is amazing, but the boardwalk, hotel and thousands of other people watching at the same time don't really contribute to the feeling of being in the wilderness. There's a stat on the National Geographic website saying that 99 percent of Yellowstone visitors never leave the park roads (I'm guessing they mean roads/developed areas). It's just amazing to me to think that people go all that way, drive around, get out, snap some pictures then go drive around some more when there's so much more to see.

I could go on and on, Montana is my second home so feel free to ask anything and I'll try to answer.