IMO< for a first time trip out west, its hard to beat west yellowstone, tons of places to stay, tons of places to rent and tons of trails and OFF trail riding
and honestly if your a TRAIL rider, you will have awesome trails and miles to ride, as most folks go out to off trail ride, so trails DON"T get beat up like they do in a more trail riding only area's
plus many many places if you do a whole package with them, give you a free tank of fuel each day to ride on!
some will even do airport pick up IF there is enough to make it worth while, other wise, if you have say 3-4 guys, RENT a car and split the costs, its pretty cheap, you will still have a little driving to to get there from any airport, about an hour or so, its just nature of the beast!
another place is Island park ID, again lots of options there too, more off trail than trail
there are other places off the beaten path better for more experienced off trail riders, but for first time trip , west yellowstone is HARD to beat IMO
all the more so if your NOT all BIG on playing off trail and getting stuck, as with so many trails to ride, you can ride all day and see awesome scenery and enjoy yourself, OR , can trail ride and stop and watch some of the more skilled or younger guns, run off trail and get stuck LOL
another idea I would even suggest, if, if you go for a week, rent 2 types of sleds, a trail sled the first couple days and then as your get more aquainted with the area, then a mountain sled, and go play off trail
keep in mind if your a flat lander, altitude out there can effect some more than others, so keep a eye out for signs of sickness
I took one buddy out and they got pretty sick the first 3 days, just couldn;t handle being up high?? (above 7500 ft)
I'd suggest drink lots of water , and know the signs of altitude sickness if you go HIGH and ain't used to it
can happen to some folks at above 8,000 ft!
its not like your on million feet up, just it effects some more than others!