This temple, just outside the city walls is unremarkable except for the Phra Sila Dam image and a nice old wooden temple building.
The Phra Sila Dam is believed to have been brought to Lamphun by Queen Chama Thewi and it has been used as a base for a popular line of votive tablets.
You can see the image seated under the umbrella, just to the left of the wooden building within the compound.
Wat Mahawan is just outside the city moat on the west side of town. If you are planning on walking out to Wat Chama Thewi, you'll walk right past it, so you may as well take a peek.