So various people I work with/know come to Boulder, may want to try a hike but have limited time. Bring water!!! These are some ideas I like:
Quick but extremely impressive:
"Back of Green Mountain" driving time 30min?, hiking time: up to 1 hr up, 20 min down, even hiking slowish: From Broadway or anywhere, go west on Baseline, right up Flagstaff Mountain (look out for bicyclists going up), right through the fee area (you don't have to pay if you just drive through), after a really long time (maybe 5 miles, very mountainous), you will pass a sign saying "Leaving City of Boulder Mountain Parks/Fee Area/something", and you will also pass the 5-mile marker. Here, you can park for free at the side of the road, either side (do not block private drives), and if you look carefully on the left (South?) side of the road, you will see a trailhead with a sign and a map. The trail will meander for a bit, and has a crossing, but the way is pretty obvious. The summit is fantastic and there is a monument at the top showing where all the regional 14ers are. It is colder than in town, as you might expect.
Quick and casual:
1) NCAR nature trail: Go to NCAR, park, walk back a bit near the entrance road until you see where to cross the NCAR drive, and go to the nature trail. Views are great, and if you want to go farther, you can actually hike through the nature trail to the southwest side, to a trail which connects west to the Mesa Trail, and you can go a long way from there.
2) Settler's Park ("Red Rocks"): Not "The" Red Rocks, but just a nice short hike in the foothills west of downtown Boulder. There is a smallish parking lot near the intersection of Pearl and Canyon streets, but if it is full, there is some street parking east of there along Pearl. Can be parked up at busy times.
3) Along Bear Mountain Road, you will see an open area on the west side with a trailhead. This accesses both Bear Canyon, which starts as a road but becomes a trail, and also the Mesa Trail, which goes along the front of the foothills. I love Bear Canyon, and again you can go as far up this as you want, if you are over enthusiastic you are likely to end up at the top of Green Mountain (my favorite) or Bear Mountain, but it is especially nice in spring/mid-summer, when there is still some snowmelt. If you go all the way up, it is colder at the top!!! (note that during June/July 2017, the bottom of this is closed for repair, but it can still be accessed from NCAR)
Other interesting general suggestions:
Cannot go wrong going to Chataqua, my favorite there is Royal Arch (far-ish), the Mesa Trail (a lot longer if you like), and the meadow right above the entrance. Gregory Canyon is also nearby and is partly shaded in the summer (parking kinda tight). These and others are mentioned here: