Best free things to do in Boise?

Hello! I lived in Boise almost 20 years and still visit at least 4x/year for business and outdoor recreation.
Outdoor Recreation City hiking/biking along the Boise River Greenbelt that is on both sides of the Boise River is hard to beat. It runs all the way up to Lucky Peak from town 25 miles in total distance. The Boise foothills has the best trail hiking around with a wide variety of terrain to choose from. Watching sunset from high above the city is pretty sweet. You can also float the Boise River for minimal cost... shuttle buses run from Ann Morrison Park to Barber Park (the put in) where you can rent a tube for a couple bucks.
City Life The Boise Art Museum in Julia Davis Park is excellent, as is the downtown area. Freak Alley Gallery, Cecil D. Andrus Park and the Idaho State Capitol Building are downtown, free and cool to visit. There are also plenty of coffee shops, bars and restaurants downtown as well. I recommend Bleubird and Jenny's Lunch Line for an easy on the budget yet incredibly tasty meal.
Out of Town (45 min - 2+ hours) One incredibly unique attribute about Boise is that it is a near perfect base camp for accessing an abundance of natural hot springs. Which, I recommend above everything mentioned above. Idaho's hot springs are not overrun like other states and come in two flavors... commercial and public. The hot springs near Boise are all easy to reach and offer an experience like none other, most of which are free to visit and use. Enjoy!

Hiking in the foothills is popular. The Boise River Greenbelt that runs along the Boise River is also a lovely place to bike, run, walk.

Walk along the river, Boise River Greenbelt. There are several museums, Zoo Boise in the area. Lots of outdoor places to have a coffee or food and people watch on 8th st.

Visit the Boise River Greenbelt, Boise Farmers Market, Boise State University campus, Table Rock all for free!

Take a short hike to Table Rock, or a drive to Lehman (hot water springs - totally free) worth the drive.