If he's interested in water sports and is comfortable in the water, you might want to consider a Discover Scuba Experience with your local dive shop. It's a basic introduction to scuba equipment and a chance to try scuba diving in a pool. Depending on the training agency the dive shop works with, there may be some variation in minimum ages and other requirements. For example, PADI (Professional Association of Diving Instructors) has a minimum age of 10 for the Discover Scuba experience and requires that everyone fill out a medical form and some releases. For ages 8-10, there is another program that PADI offers called Bubblemaker. It's fairly similar to Discover Scuba but has a maximum depth of 6 feet. So, it may require some advanced planning and communication with other parents. Even so, this is probably one of the coolest things that kids can do for a birthday party.