There's been three or four threads on Mill Creek, run a search and you'll find a lot of info. If you enter from Combs, there is a trailhead with a mapstation, right next to the creek. There is a ton of camping all along this creek. If you unload here and explore a little, I'm sure you can find something out of the way with a little privacy. It's been a while, but I remember three or four small (4-10 ft) waterfalls in different places (don't remember how well the creek runs in the late summer though).
There are MANY miles of trails out there. Marked and unmarked. If you rode continuously ALL day, you'll cover all of the marked trails and some of the unmarked trails. I don't think you'd be able to see all of it in one day, it just depends on how well you are able to spot the branches off the main trail. If you enter from Combs, the main trail complex will be in the mountains to your left, but there are many trails to the right (all unmarked---but still very long). Also, at the top of the mountain along the dirt roads there are many more miles of trails. I think there is plenty there to keep you busy for two or three days. Regardless, I don't think you'll be disappointed! It's beautiful up there, there's single and double track all over the place, some VERY challenging sections and not overcrowded! Enjoy!