If you don't have any plans yet, the Swan/Teton Valleys have a lot to offer. From miles of technical singletrack to the St. Anthony sand dunes just an hour away.
Sandwiched between the two valleys are the Snake River Range, and the Big Hole Mtns. There is easily a couple of days worth of riding here and various lodging and camping options in Swan Valley, and the Driggs Victor area. i cant find my map but I think it was a trail/mtn. bike guide by Beartooth Publishing.
http://rockymountainmaps.com/images/...on_Jackson.jpg . This one worked pretty good along with a USFS MVUP.
The sand dunes are a lot of fun as well. 10,000 acres of sand to explore and carve. Just make sure that you have a flag on your bike.
As far as bikes go, any 250-450 4t or 2t will be great on the ST. You might want a bigger 4t fore the dunes, I didn't have my 300 when I lived there but I'm sure it would be fun. I did have a bad experience on a wr 250 though, the clutch was not happy.
And as far as plates go, Idaho is pretty easy going. The cops won't hassle you for riding on the road as long as your not on an interstate hwy., at least thats what the local cops told us
