Friggin' awesome area, I've spent a good deal of time up there working. The ski area has some killer mountain bike trails, there are dirt roads all over the area for exploring... and I'm sure there is riding, but I don't really know where to send you. Best bet is to find a map and check out roads that look interesting. Perhaps call a power sports place about the trails... I know there's a shop in Fraser, right outside of Winter Park.
Fishing the rivers & creeks is prime up there, I've caught plenty. Make sure you know the rules regarding fishing, you're at the headwaters of the Colorado River up there and there are very strict rules regarding bait, catch and release, etc.
One thing you really should do, grive to Granby and head up into the Rocky Mountain Nat. Park... bring your camera! You'll see some massive Elk, Moose, Big Horn Sheep, etc... there's a ton of wildlife up there. The highest paved road in the US is also up there. Bring a jacket, it will be cold!
Here's some 4x4 trail ideas.... go to Trail Damage, look around Fraser... there are several in the area. -
http://www.traildamage.com/google_colorado.php
I already mentioned it, but... It's COLD up there... I was working up there in July, several years ago... and I had to scrape ice off my car in the morning.
