Bonnie, we've been to Catalina on cruises 3 times in the past two years. We've never taken any of the arranged excursions, 'cause it is so easy to do on your own.
Here are some ideas:
* A short walk from the tender pier is the Casino Ballroom and Theater. This is a great tour for $9.95. You see the theater, an old time movie about Catalina, a tour of the ballroom, and free admission to the museum on site. Very neat and fun.
* You can rent golf carts near the center of town. If you rent before noon, you can get 3 hours for the price of 2 hours, which comes to about $80. This is great if you have another couple with you, too. The cart rental place gives you a map of sites. Drive it up to the Wrigley Memorial and the Bontanical Gardens. Go on this windy road around the island for some great vistas. We saw it all and still had about 15 minutes to spare. After bringing the cart back, we did some shopping in town. (By the way, you can stop at the tiny Vons grocery store in town for any groceries you may want to bring back on board.
* You can book a tour right on the main green pier, too, if you want a narrated talk (much cheaper than the ship, I'm sure.)
* Finally, for the least adventurist, Catalina's downtown is very quaint with a no-traffic promenade.
We've never done any snorkeling, para-gliding etc -- just not our thing.
We did the long bus tour one year (the one that takes you to the airport and museum) and I wouldn't advise anyone to do it because it is long and boring. One year we played miniature golf and had a blast.