If you want to condition for sand volleyball, there's a few different aspects.
The first is cardiovascular fitness. You can do this at the gym if you'd like, on the Stairmaster, the stepmill, the treadmill, the elliptical. You could run, jog and do sprints along the beach.
It's important to not only do cardiovascular but also interval training that's more anaerobic. Volleyball's a game with only a few seconds of exercise with frequent breaks. A very long rally is usually not any longer than 15 seconds. It's also important to train with strength training.
You can do this with weights or you can do this with body weights. You can do a lot of single leg squats and different plyometric exercises as well to get in good shape for jumping.
It's important also to train during different times of the day. If you're playing in a tournament, you don't know if your first game is going to be at 8 in the morning or 11:00.
Be used to the sun and the wind and the cold. Even the rain. A lot of times we train when we're fresh, when we have a lot of energy. But it's important to also do technique drills when you're exhausted because you still have to maintain your technique late into a tournament. You may have already played 10 or more games, but you still have to maintain that technique to win.