First of all, figure out what dad might enjoy that you can do as a whole family, and consider a few of these options:
1. Rent tandem bikes or bikes with child carriers and take a family ride. Check out rentabikenow.com for local bike rentals.
2. Make it a day all about dad. If you live in the town where he grew up, take the family on tour of his boyhood home,school, and playground, making maximum effort to make him the hero of as many stories as you can collect. Plan a stop on the way to have lemonade with grandma, if she is local.
3. Take the family golfing, mini-golfing that is, and bring his putter so he can really have a shot at winning.
4. Visit the zoo, but talk less about the moms and babies, and more about what the daddy animals do to protect and play with their families. Ask the kids what role they think daddy elephants and tigers play in their families. You might learn a lot here.