Depending on your physical attitude on Catalina you can rent bikes or you can rent a golf cart and see the land, there isn't much of it. most of what to see is right there at the mole', the area that your tender will dock at, most cruise lines offer a tour, if you want to call it that, it is actually a bbq and beach blanket bingo that goes to Descanso Beach which is just beyond the Casino, which you can definitely tour both on your own if it interests you. Past that you don't really have to do ships tours, the big deal is the sub ride which you can book yourself, through the Catalina Island Tour Company, they are the only game in town. Check out the link for tours on your own and if your interested in a last drink before leaving town don't pass up Luau Larry's on the main drag
http://www.visitcatalinaisland.com/avalon/Tours.php