I'm new to this off roading also. Found these excellent guide books titled
'Colorado Trails, backroads and 4-Wheel drive trails' by Peter Massey, Angela Titus, and Jeanne Wilson. Specifically the Central Region or Front Range Region editions. Go to The Tattered Cover on East Colfax. Just what you are looking for.
Mt Biking: White Ranch just north of Golden, Apex Trail, Matthews/Winters Park at Red Rocks, which is also a good hike. Favorites about 1 hr drive off 285 are Kenosha Pass and Buffalo Creek. Ask at bike shops, ask other bikers what their top 3 are.
REI Flagship store downtown: ask employees their, especially the ones downstairs in the rental dept.
Oh yeah, snowmobiling at Winter Park or Grand Lake. Unguided rentals. So much fun.