Catalina is expensive, no doubt, but worth it!
You can catch the boats from San Pedro, Long Beach or Dana Point (Orange County). If you're trying to make a flight to/from LAX, I'd recommend not going from Dana Point - it's a longer boat ride and further from LAX.
When you get off the ferry in Avalon, you'll want to head right towards the town. Follow the walkway along the water (Green Pier will be on your right the whole time). About a block down on the main street, there is a Visitor's Center, where you can purchase tickets for tours, glass bottom boat, etc... If you continue down another block, on your left is the place to rent golf carts. Avalon proper is pretty walkable - it's only if you want to leave the "flats", and head up into Wrigley Botanical Garden, or the hills above the flats, that you'll need a golf cart (and even then, some of those roads are not accessible to rental carts).
Personally, I get really seasick so glass bottom boats are out for me.

But my DH and kids love the semi-submersibles (yellow subs). If you're only there for the day, I do recommend taking the Avalon City/Overview Tour (by bus), doing some kind of boat tour, and the Casino Tour (the Casino is not a gambling casino but it is an absolutely beautiful structure worth seeing).
If you'd rather kick back and enjoy the island, continue walking along the main street, toward the Casino (big white round building with red roof - can't miss it). Pass the Casino, and just a bit down is Descanso Beach Club. Get a seat under a palapa, order a Buffalo Milk, and enjoy.