M.G.
K. - I've had really good luck doing family trips by using a website called Homeaway.com. Its a national website that lists houses/condos that you can rent for vacations. Its way cheaper than getting hotel rooms and you will have a kitchen, family space etc. For our spring break we took my parents, my sister and us (family of 5), to Tucson where we stayed in a beautiful home, surrounded by gorgeous desert in bloom, with our own swimming pool/spa. It was great!
As for SW Michigan, I just spent a long weekend in Coloma Mi right on the lake. I don't think you can go wrong with any of the obvious choices: Union Pier, South Haven, St. Joseph's. I would really try to find a house that you can enjoy (pool???). Good Luck and have fun!