After you book a cruise, most lines will send word advising you to sign up immediately for shore excursions. They'll tell you these trips fill up quickly and you need to reserve space right away to avoid disappointment. Look at the prices carefully, because many times they are inflated. It's often possible to step off the ship and make your own arrangements at a substantial savings. These trips might not be quite the same as you'll receive through the line, but many budget travelers reap great savings by doing a little research and booking outside of the cruise line offerings.
Avoid overpriced shore excursions by making your own arrangements.