Lucky you. It's fun to have several days on the island so that you can do both diving and non-diving activities.
If you're spending a week on Catalina at Avalon, you might get the chance to dive the Valiant wreck during the middle of the week when there isn't any boat traffic. Ask the Harbormaster for permission and directions to check out the wreck. It's a fun dive. With the right compass headings, you can navigate underwater all the way back to the UW Park and burn the rest of your tank in the kelpbeds.
If you don't mind the extra expense, I'd recommend doing a dive boat at least one day during your stay. You'll see similar critters to what you might see in the UW Park. You'll see giant sea bass, sea lions, morays, several kinds of rockfish, lots of nudibranchs, etc. Last time I did a Catalina boat dive, I recall a fantastic afternoon at Middle Reef where we saw a number of leopard sharks weaving in and out of the kelp. The rays of sunshine were streaming through the kelp, and the vis was so nice it looked we were diving in an aquarium! On the first dive that trip, we saw several "gangs" of giant sea bass. Pretty neat stuff. A 3-tank dive boat experience at Scuba Luv (King Neptune) costs approx. $120 per person. At least 6 divers must reserve spots in order for the boat to make the trip.
Some of my favorite dives at Catalina are sites close to Two Harbors: Ship Rock, Bird Rock, Middle Reef, etc. If you are staying at Avalon, the only way you'll ever get to dive these sites is by going out on a dive boat.
Have a great time!