Mt Biker? Road Biker? 4x4 trails?
Welcome to Colorado
There are some real good trail guide books just about every store.
Pick up a copy of Funtrecks. You can also use Traildamage.com for trail information. Keep in mind that the rain we had this last summer has changed many of the trails so the information available may not be totally accurate.
You may want to beef up that pretty 4Runner before hitting too many of the trails. Skids, slider, lift and tires would be good. RCI is just up the road from you, about an hour and half drive. Slee is over in Golden for custom work and Toytec is up in Thorton, about 30 minutes or so. Discount tire is everywhere. Firestone has lifetime alignments, I paid $139. Even if you don't hit the trails, the pot holes here justify the lifetime alignment.
The weather here changes quickly so do some research on being prepared for cold weather. Also the sun is very intense so don't forget sun block.
Ice is not your friend so respect it a lot.
I know there are a lot of bike trails up in Denver but being a fat old fart, I don't take advantage of that. Keep in mind the air is much thinner here.
Altitude sickness is a real thing so drink lots of water.
Hope to see you on a trail.
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.