Although the whale sharks are the main tourist attraction, there's much more to do in the region, and I recommend taking an island-hopping boat tour one day and a firefly tour in the evening. Philippine fireflies are much smaller than those found in the US, and the sight was not quite as incredible as when the fireflies decorated the park last sumer in Falls Church, VA, but it's worth taking the tour.
We stayed at Giddy's, a Filipino-style hotel I recommend despite having to chase the kitchen staff down in order to be fed--every single time. The cooks were on island time. Other than the restaurant, the hotel staff was very good about organizing tours and being on time--a rare experience in this country, especially in the provinces. Another rare experience: the hotel was set in the middle of the town, so we were able to witness and even participate in local life without the barriers of guards, gates, and metal detectors. The night before we left for Donsol I returned from two nights/three days in Singapore, and, having had some time in Singapore to recover from Manila, I fully enjoyed this unique experience. Had I not been coming from the cleanest, most efficient city in the universe, I may not have been able to appreciate Donsol quite as much.
When you land at the Legaspi airport and step outside the plane, you can't miss the Mayon Volcano--a very active volcano.
The hour long trip to Donsol takes you through villages and lush greenery, and depending on your vehicle, is a pleasant trip. If one day of swimming with the whale sharks is enough for you, take an all day banca trip for snorkeling and waterfall swimming.
And finally, the whale sharks. My husband's description of the experience:
This is exactly how it happens -- the guide jumps in from a moving boat, you follow, knowing not where the beast lurks. You frantically search around in the dim water, looking for the city-bus sized creature. When the guide stops swimming, you know it's close. Then BAM ... there it is and you better move out of the way.
Even though the whale sharks are plankton-eating gentle giants, one dive was enough adventure for me.