When we lived here, we had an annual membership to Discovery Gateway Children's Museum, which is the best deal ever. This is one of the better children's museums that I have ever visited. With two floors of interactive play, it's easy to spend hours exploring and playing!
And since you'll already be downtown, then why not go get the most authentic Japanese food that you can get outside of Japan?! Seriously. Koko Kitchen is the best. My husband, having served an LDS mission in Japan, and growing up eating his mom and grandma's delicious food, is always in search of good, authentic Japanese restaurants, and we found it in Koko Kitchen! (Sorry I didn't take any photos, but be sure to click through on the yelp link pictures!)
What are some of your favorite things to do in Salt Lake? We also love visiting Hogle Zoo, The Living Planet Aquarium, hiking up Ensign Peak or Adam's Canyon (both kid-doable!), biking up Emigration Canyon (not so kid-doable) or along the Legacy Parkway Trail (great for strollers and bike trailers), and shopping at City Creek Center or at Gateway Mall and playing in their fountain in the summer.