jharris-
if you're still planning on doing this, we might be in the area at the same time if you'd like to do a route or two with us on a weekend.
mtb really depends on what you're in to--xc cruising, dh, stunts, tech, vertical, or a little of everything. ask the guys in the bike shops, they'll be able to steer you to a good mtb trail for your skill level, riding style and equipment.
if you want to avoid the masses, hike the devil's garden loop backwards...tho this time of year, you'll be avoiding the masses anyway. the needles district is great for hiking as well, there's probably more backcountry solitude there than anywhere else in the easily accessable park, and it starts at the pavement so you can park the rig.
northwest of moab is mostly easier routes, with plenty of hiking and mountain biking. you can mountain bike anywhere on established routes, there are a lot of choices.
if you havent been before, the klondike bluffs route is a good one--4wd, mtb and hiking all in one if you like. don't take the route over from klondike to the salt valley road with a stock truck unless you're very experienced offroad, know the vehicle very well, and have plenty of belly clearance and skid plates...it's full of ledges, holes, and boulders. we brought a stock tundra through, so you might be able to make it, but it is rougher than the klondike bluffs or tower arch routes. if you can make it over, there is a hiking trail from the salt valley road between klondike bluffs and marching men that leads to tower arch.
definately post if you're still headed this direction, maybe we can meet up in moab.
-sean