If you're looking for an easy hike without a lot of elevation gain, consider the Greenbelt Plateau Trail. The trail gains just 200 feet of elevation in 3 miles and it features views of the flatirons and the Boulder valley.
The trailhead is about six miles south of Boulder off Broadway/Highway 93 (directions below). Find a parking space, walk through the gate and take a look at the map. The single-track dirt to the east is the High Plains Trail. The wide, road-like trail to the north is the Greenbelt Plateau Trail.
Start walking north on the Greenbelt Plateau Trail. As you hike, make sure you don't just look at your feet on the trail. Look up. Look east at the rolling plains. And look west to see the beautiful views of the foothills and the Boulder flatirons. There are a few scattered trees in the area to use in your photographs. However, there are not many trees near the trail, so don't come on a hot, sunny day.
About 0.8 miles from the trailhead, the trail comes to a "ridge" of sorts. From here, you can see into Boulder valley. From here, the trail turns slightly left and begins to drop. You can turn around here for a 1.6 mile roundtrip hike or continue on. Remember, for every step you take downhill, you'll have to climb this on the way back out. If you continue on, you may spot a large lake to the east. That's Marshall Lake, a private lake.
At 1.2 miles, hikers come to a trail split. You can turn right for the Community Ditch Trail and a loop around Marshall Mesa or continue on the Greenbelt Plateau Trail. Quite honestly, the Greenbelt Plateau Trail only goes another quarter mile or so from here and it ends at a very busy Highway 93. I would suggest turning around here. Or if you want a longer hike, take the Community Ditch Trail to add a little more distance.
Details: The hike to the overlook and back is 1.6 miles roundtrip with about 35 feet of elevation gain. The hike to the Community Ditch Trail split and back is about 2.4 miles with 100 feet of elevation gain or so. The hike to the end of the Greenbelt Plateau Trail is 3 miles roundtrip with about 200 feet of elevation gain.
Directions: The Greenbelt Plateau Trailhead is 0.1 miles east of the intersection of Highways 93 and 128. In Boulder, take Broadway south. The road becomes Foothills Highway/Highway 93. Turn left/east on Highway 128/120th Avenue. Go 0.1 miles to the parking lot. (You will have to make a U-turn to get there.)