Some good replies so far. Here are some additional suggestions:
1) stay in donsol or legaspi? if so any recommendations?
-> As mentioned above, definitely stay in Donsol for the whale shark interaction. It will be extremely difficult to travel back and forth. I stayed at the Amor Beach Farm and can recommend it. It is only a 15 minutes walk from Amor to the Whaleshark interaction center (or a 2 minute trike ride). I communicated with Rasyl (rasyl_r_amor at yahoo) but ultimately just showed up and got a room on the spot (the last one they had available). Other people like the Vitton Resort,adjacent to the whale shark interaction center. It is a little more expensive.
If you want to check out the volcano, you might consider spending one night in Legazpi City.
-> As mentioned above - Cebu Pacific flight from Manila to Legazpi. Once in Legazpi you can either share a van with other tourists straight from the airport, or walk outside grab a tricycle to Terminal and take the public van to Donsol (leaves when full, so you might need to wait 20 minutes) - a bit more hassle but cheaper.
3)can you get on the trips or excursions without booking ie turn up?
->Just turn up. No need to book whale shark interactions in advance. But you will need to be there early enough to make the morning interaction or sign-in in the afternoon and go out the next day. Things can be a bit chaotic there in the morning, and you are required to watch a short video. They will match you up with a boat so you can share the cost, or you can just ask around and find a group to join.
4) How many days you ideally need from point of arriving in manila to being finished in donsol if a days trip with the whale sharks was enough?
->When I was there in mid March they only had morning excursions, so I couldn't have made it out the first day. Two additional days of interactions were good enough for me. I also did a third day of diving in the manta bowl but failed to see mantas.
Other things to consider:
-Check the calendar and avoid traveling during Holly Week (around Easter).
-Allocate some time to check out Mt. Mayon (see photo on Kevster's trip report above). This is better done from Legazpi City itself and not from Donsol.
-Internet access was only available in Donsol village
-If you're sick of the resort food go into Donsol village for a meal
Enjoy your trip.