This trail is part of the Niwot trail system that connects to trails headed to Longmont and Gunbarrel. It's a quick loop, but there are several turns, detailed below.
The lower part of the Niwot trail is remarkable in that it passes very close to the center of Niwot, and right through a major subdivision, but doesn't feel like you're anywhere near a town.
The trail winds up from the trailhead just off of N 79th St. and crosses a bridge over an irrigation canal almost immediately. Bear left after the bridge (right take you to the upper Niwot loop) and the trail cuts through a wooded section before popping out on a very short section of paved path (20 yards).
When you hit the pavement, follow the paved path straight ahead - you'll see a beach volleyball court on your left. This is the only tricky part of the directions - after the beach volleyball court, cross a small wooden bridge, and immediately look for two side by side singletrack trails on your left.
Take either one of those singletrack trails - they'll look less like a trail than the rest of the loop, but they wind through the woods and end up at Longview Dr. When you reach the road, make a left, and follow a paved bike path to Niwot Rd.
Continue on the paved path along Niwot Rd., cross a small residential street (83rd St.) and keep going until you hit another dirt path. The path continues back across N 79th St closer to the center of Niwot.
Follow the trail until you see a left hand turn (turn left), pass two small ponds, and then cross Monarch Rd - there's another trailhead just a cross the street. From here, follow the path through a couple of cattle gates, and stay left at the junction after the second gate.
You'll cross a wooden bridge, then stay straight to get back up to N 79th St. Cross the road here and you'll be back where you started.