I love Lake Harriet because each lake has it's own feel to it. Lake Harriet is more family oriented just because there is the bandshell there. If you look during the summer, free movies in the park are great. There is even a website:
Lake Harriet is calmer than Lake Calhoun, but again is family oriented. If you want to avoid some traffic, I suggest walking there. There are sailboats and a couple summer beaches. But, I love to walk to the bandshell and get ice cream, popcorn, and then listen to the free music in the summer. The best part, is you can bring a picnic and watch a movie or listen to music and it's in the open air and FREE!