Taking Super Shuttle to Boulder is over 90 minutes each way, and it doesn't look like it takes you to any place close to Pearl St.
Super Shuttle to downtown Denver is an hour each way. If you do take this, I would recommend either being dropped off/picked up from Westin/Tabor Center (or maybe Brown Palace). It's a Sunday afternoon with no baseball game in town, so downtown will be very quiet. Well, there is an Arena Football game at 2pm, but why spend your time indoors?
Here's a link to what to see if walking around downtown...
http://www.denver.org/visitors/walking.asp
And if you're so inclined, you could walk two miles from Tabor Center and ride a few
roller coasters.
Have fun in Denver.