One of the things I enjoy about the cities in this area is that they all take their turn. Each night of the week, West Linn, Lake Oswego, Tualatin and Wilsonville all have farmers markets and concerts in the park.
Some are more fun than others. West Linn certainly does a wonderful job organizing the concerts. Tanner Creek is an excellent location. They dedicate one part of the field to temporary parking, there are a couple food booths, games for kids and a nice stage, but something is missing.
I can say this from all my years going to Tualatin's Concerts on the Commons and Wilsonville's occasional concerts in the park. There is more audience participation in the other locations. More cowbell.
Music in the Park is every Thursday night for about eight weeks in the summer. A mix of artists, including Curtis Salgado and soul and latin groups. The place is absolutely packed and parking can be a huge hassle if you arrive late. If you have kids, they can enjoy the water feature (bring their swimsuits) and playground. There are only a few food carts (BBQ, snow cones, pizza), so you would probably do better bringing a picnic from home.
So a good time if you are in the area, but sadly the 'Keep Portland Weird' sentiment hasn't extended to my burb.