There are locations where you have to pay admission, but will simply have so many choices that I'm sure you'll find something to make the admission worth it. Try the Grohmann Museum roof top garden, Villa Terrace, Lynden Sculpture Gardens, Mitchell Park Domes and Boerner Botanical Gardens. The Milwaukee Zoo has some great spots too.
|The rose garden at Boerner Botanical Garden|
|Lynden Sculpture Garden has a pond and a variety of trees|
|Villa Terrace has blooming flowers, an arbor, staircase, nice patio and fantastic fall color|
|Big Bay Park|
|Bradford Beach in January at sunrise|
|Lake Michigan at Grant Park|
If you like the huge white rocks, Big Bay Park in Whitefish Bay is great. McKinley Beach has some pretty rocks off shore that add to the interest in a lake photo. You can actually climb on them at McKinley to get both the rocks and the lake. My favorite park for rocks, water, bridges, and beautiful flowers, grass, etc is Lakeshore State Park. It has everything you need for outdoor shots. If you come late in the day, the lighting is awesome as the setting sun reflects on the water side of the park.
|Put your model on top of the rocks at McKinley Beach|
|Rocks at Big Bay|
|Lakeshore State Park|
|View of the lake at Lakeshore State Park|
|View from Kilbourn Reservoir Park|
|View from the Healing Garden at St. Luke's Hospital|
|South Shore Park|
|Larger trees are fun to use|
For bridges, well, you can probably guess that I'm going to recommend Seven Bridges trail in Grant Park. I also love the covered bridge over the ravine at the trail head. The bridge over the river between the Holton Street swing park and Lakefront Brewery is interesting and you can get views through it as well as river shots.
|Trailhead at Seven Bridges Trail in Grant Park|
|Stone bridge near the waterfall at the Mill Pond at the south end of Grant Park|
|Holton Street Bridge|
|Dragon at Cass St. Park|
|Mural at Boys & Girls Club near Cass St. Park|
|River Walk has interesting structures and water|
My favorite photo of my youngest son was actually taken at Holton Bridge Swing Park. That's a good place even if the sun isn't shining because it's covered. There are a variety of swings and the textures of the bridge overhead make it interesting.
|Holton Swing Park|
|Labyrinth at Alice's Garden|
There are likely many more spots. Add your great finds in the comments!