Denver is a beautiful city, especially if you're visiting during the summer. It's not too hot or too cold and the scenery is outstanding. Every day it would rain around 6 p.m. and cool everything down. Then the sun would come back out just in time for an awesome sunset.
If you're not that interested about what the capitol represents or what goes on there, at least climb to the top of the gold dome. You'll get an excellent view of the city.
The Denver Art Museum has fairly good collection and is definitely worth an afternoon visit.
The Renaissance Festival will transport you back in time. Pretend you were a fair maiden or a knight in shining armour.
Denver Mint--Money makes the world go 'round. Learn how they make it. Buy some too.
The Denver Museum of Nature and Science is great for kids and has plenty of hands-on exhibits.
The 16th St. Mall is full of street performers and lots of shopping. Buy your cheap souveniers here.
Quick Tips:
It may be hot in the summer during the day, but you better have a jacket and pants for the evening. An umbrella wouldn't hurt too since it rains just about every evening, but the sky clears up shortly after.
Best Way To Get Around:
Drive if you must, but ride the bus if you can.