The Manta Ray Snorkel trip is amazing, but you said you are on a budget and it's pricey. A good second choice is seeing the Manta Rays for free at the Sheraton. You need to arrive about dusk, then park in the parking lot and follow the sidewalk around the hotel on the right - staying close to the hotel. Eventually you will come to some steps that take you right up to the observation platform without having to walk through the hotel. We could see several huge manta rays feeding on the plankton attracted by the light - it was great! (And FREE!)
We went on a great booze cruise out of Kona as well. Great appetizers, and generous, all you could drink, live music and hula. It was so much fun!
If you go to the
Place of Refuge (and you should) you really have to snorkel there, even if you don't snorkel any where else. Right in the next little bay there is a natural lava bench called
Two Step and that little bay has the best snorkeling and turtles I've ever seen and we snorkel a lot! Take a picnic lunch and go early - plan to make a day of it. Park in the lot for the Place of Refuge (it's a National Park so there's an entry fee.) We parked right by the ranger shack so that we didn't have to worry about the stuff in our car while we snorkeled - well worth the $5 park admission fee. Then walk back out the short driveway to the road, turn left and take the next driveway back toward the ocean. You will see people out on the lava bench kind of lined up to get in the water at Two Step - a natural step set into the lava bench that make it easy to get in the water. The snorkeling there is awesome and the place is crawling with turtles. After you snorkel, go back to your car and drive to the opposite end of the parking lot and then take the dirt road that leads off the left. It will take you to a beautiful picnic area right on the ocean with picnic tables and restrooms. If it's still pretty early, tour the Place of Refuge, and then come back and eat your lunch. If not, have lunch and then tour the park. We went back 3 times in 2 weeks we enjoyed it so much - and if you keep your receipt the admission is good for 7 days.