Well, my in-laws live in Venice and I've been dozens of times. My child is 5.
Things to do: Ice cream in downtown Venice.
There is a fountain in downtown Venice which sprays water.
Other things are further away:
Sarasota: Mote Aquarium
Sarasota: Jungle Gardens
Tampa: Busch Gardens (1.5 hours away)
Dinosaur World (near Plant City 1.5 hours away) -- totally worth it for this age, we have been twice
I have not been to Lego Land, it is further than Dinosaur World, but not by much.
We've been to the big cat rescue in Punta Gorda, FL
We went to Sailor Circus, but that is only a couple times a year.
Check out tripadvisor. I don't recommend the Ducky tours, total waste of money. I reviewed it for Trip Advisor.
Siesta Key - Sarasota beach
Fort Desoto Nat'l Rec Area (great beach)
We always go canoeing in Oscar Scherer State Park.
We are starting to explore south of Venice to Fort Myers.
We had a nice time in the Imaginarium (Children's Museum in Ft. Myers)
We saw manakees at a part south of Venice. Summer will not be the time to stop there, that is a winter, spring thing.
We walk down to the local beach and play in the sand and waves. You can bike, if you have bikes.
Is that enough?