Depends what you are into.
Capitol Building (not sure if they are still renovating it)
16th Street Mall/Larimer Square- outdoors restaurants and shops if the weather is nice
Coors Field &/or Mile High (Broncos)- not sure if they are open to public
US Mint- not sure if they have tours
all of the above (except Mile High which is just the other side of I-25) are in the downtown area so you can walk and/or take the free 16th Street Mall bus from one to the other.
Bass Pro Shop (Huge- part of the Northfields Outdoor Mall right off I-70)
Cherry Creek Mall/Shopping District
Museums (I think the Natural History has the Imax theater)
9 Holes of Golf if the weather is nice
A variety of Parks/Rec areas
Boulder/Pearl St- a bit out of your way but doable
Coors Brewery (in Golden, never been myself)