The Baja, is a good trail for being in the city, right along the Menomonee River Parkway. It is not very long but there are many different trail choices. In the middle of the trail there is about 9 or 10 dirt jumps that are well maintained. A fun ride if you have an hour or two to kill.
How to get there:
From Milwaukee, take 76th Street over the Harmony bridge. Stay in the left lane and keep left, both lanes go right and there is a small land to turn left, turn left go down the hill and cross the railroad tracks. There will be a long parking lot go to the end and bike along side the rail road tracks for about 100 yards and the entrance is on the right.