Boulder Creek Path

Oh the places you'll go, oh the things that you'll see.
Normally I'm a big fan of hitting these trails on weekdays when they're the path less traveled. However, you may want to plan your Boulder Creek Path jaunt on a Saturday (morning) so you can catch the Farmer's Market. It's located on 13th Street near Central Park, but if you stay on the trail, you won't miss it. In fact, you'll probably have to get off your bike to negotiate the crowd. This would be a great time to catch a healthy snack or just enjoy people watching.
You can access the trail at many spots in Boulder. Instead of boring you with lots of locations and details, let me just suggest that you find Boulder Creek. Once you locate the creek, you're not far from an access point.
I prefer to start at Stazio Fields, east of Boulder. From there, it's easy to stay on the trail and follow Boulder Creek. Unless there's some kind of scheduled maintenance, you'll never have to go on a road, even though you're venturing through the heart of downtown Boulder. In addition to a creek, lots of shade, and some of the most interesting people you'll ever meet on a trail, you'll have lots of options for side trips. The trail goes right by ponds, a library, tennis courts, hotels, bike shops, and not far from the incredible Pearl Street Mall.
The best place to make a rest/diversion stop is the Boulder Library. It is located right off the trail. At one point, if you keep going straight, you will ride your bike directly into the library. (I don't recommend this course of action.) Besides the obvious activity of relaxing and reading books, the library makes a great stopping spot because: 1) It makes a great people-watching spot. 2) There are benches and drinking fountains nearby. 3) It is a great spot to make a detour to Pearl Street Mall. 4) As far as exertion goes, this is a good midway spot.
After your brief respite at the library, you're back on the trail and soon going by the Boulder County Courthouse. I won't give you a link to that, because hopefully you won't need to spend any time there. Upon passing the court house, you enter Boulder Canyon. For the next couple miles, you will be within the narrow confines of the canyon walls. The creek gets a bit more restless, the uphill gets slightly more tedious, and the clientele gets a bit more athletic. From this point, you start seeing more runners, racers, and rock-climbers. You have left behind the vagrants and commuters. You will cross under the highway a couple times before you get to the end of the trail.
Take a short breather, enjoy the fresh air and sunshine, and then begin your descent back into Boulder. Be careful of your speed. It's easy to build up some momentum here, but resist the urge. The trail is windy and often heavily traveled. Due to their numbers and reputation, bikers are already on tenuous footing with drivers and pedestrians in Boulder. If you are going too fast, at the very least you may get the evil eye or an earful of "advice." At the worst, you may cause an accident. So relax and enjoy the ride down.