hey seann,
There are many things to do from Snorkeling to Whale Watching. Since your children are 3 and 7 and you're doing some fishing, I would recommend the Whale Watching or even a Boat Tour. If that's a bit much, why not hit the beach? Every kid loves to build sand castles and play on the shore. After, there's always some local shopping. (Not sure if your children are boys or girls)
If you need a few more ideas check out the
Puerto Vallarta. There could be some nearby towns that you'd want to include in your trip.
I hope that helps!