Im not a usual here on the forums... but I live in downtown and thus can comment. Car is pointless and will just slow you down. Everything you need to see is within a few mile square area. Stay downtown, and get a bike pass for the time your here... http://www.denverbikesharing.org/
This is the best way to see the city. You can get everywhere downtown easy.
Things I like:
-Cherry Creek (Great ride!!) Take this north too Confluence Park
-Museum District (Art Museum area.. Nice architecture)
-Commons park (CPV valley - Millennium Bridge)
-Lodo (Historic area)
-16th Street Mall
-Mint (Gov district)
-Union Station (Major construction going on here.)
-Capital Hill (Older more historic residential area. Very diverse, fun)
-Larimer Square (Larimer and 15th) (Also see other side of Larimer at around Larimer and 20th)
.. If you like beer: Wynkoop Brewery, Great Divide Brewery (I would suggest getting on one of their tours.), Denver Brew!
- Impulse Theater (Great improve show!, cheap too (In the basement of Wynkoop found in LODO near Union Station)
and much, much more.
Lots of interesting architecture to see! Have fun!
Everything mentioned is within a 15 minute bike ride of each other and there are bike stations all over the place!