We blew off TSI for years. It's now one of our favorites, for several reasons:
A) on a hot day in a park with little shade, it has trees, and places to get cool without that bone-tingling AC.
B) There's much to do.
C) It's seldom crowded.
The "to do" list.
The caves. The neatest thing, something a lot of folks miss. There's two caves, real honest-to-goodness exploring-type caves. They're cool without being chilling, and have many mysteries and surprises along the way.
Fort Sam Clemens is on the far end of the island. A gated fort, there's plenty of stuff for kids to do while parents sit and rest, including manning the fort guns (non-offensive munitions). Note: this adult loves firing the guns. There's three levels: the upper level (the most action), the ground level (including a livery with very realistic horses), and the "escape tunnel" in the basement.
There's several bridges on the island, including the barrel bridge (which I could do without). There's trails (nice, quiet nature trails), and Aunt Polly's, a quaint restaurant where you can get simple fair (the Peanut Butter & Jelly Sandwich is a neat touch). And on the island you can get some neat views of the BT/SM attractions, the Haunted Mansion, and the Riverboat as it goes by (all good photo ops).
Our average time (ferry boat included) is about an hour, usually over lunch.
WARNING: Many people feel the way I do about TSI. The one's that don't seem to hate it with a passion, calling it a "waste of time." I think they're nuts, but they think I'm nuts. Read a few reviews and figure out who YOU think is nuts!
Pat