Catalina was my absolute favorite place on Earth growing up. The Holly Hill House was my dream home

. Avalon is really the only place to walk around and it's small enough that you don't need to pick and choose where you go too much. Just strolling along the waterfront is sooo nice, especially in the morning when everything is cool and quiet with the sun rising and the bell tolling. Aahh... The smell of freshly baking ice cream cones.... You'll have a nice time.
Avalon has some good dive spots. If you haven't been diving yet, it's a good place for an introductory experience. If you have been diving, you can check out the dive park. If you're not into scuba diving, you could just take the glassbottom boat ride. I live in Hawaii but the waters around Catalina have so much more life and beauty. Definitely worth a look.
Taking the night boat to see the flying fish is fun, too! But even if
everything is too much money, you'll still have a great time just
being there. Granted, it's been a while since I've been there, myself, but I can hope all it's beauty has been preserved through the years.