Temps today are in the low 70's but snow is expected tomorrow and Wed. with temps in the low 40's to low 50's for the week.
From downtown you have access to the Cherry Creek, Platte River Trail or the Clear Creek Trail. All are safe, easy to find and ride.
I haven't ridden all winter but I'm sure the trails are clear and free of snow.
The DBTC (Denver Bicycle Touring Club) http://www.dbtc.org/ has a library of routes and maps on their website or you can pick up a DBTC map at any bike shop.
I'd offer to show you around but this is ski season and I will be in the mts next week.
Good luck and have fun.