Like vanilla_bean, there are lots of activities in and around Manila.
However, If money is no object, you can easily fly out to any of our islands early saturday morning and fly back last flight sunday. Yes there are numerous spots close to Manila that is accessible by car, but if like me, you would rather fly than spend time in traffic, remember the airport is very nearby Makati and Manila, wherever you might be staying.
Check out a similar thread on the best weekend adventures from manila:
As for the diving, I took my certification course in Boracay for 6 straight days, so I was there about 8 days total. But years before that, I usually get the intro dive which is a non certification dive and a basic, easy dive with a divemaster not leaving your side almost the whole time. If you dont have that much time and you'd want to experience diving, then its a good option. Better do it on the second day when you have at least 3 days/2nights in Boracay, Bohol or Palawan. As we were taught not to dive and fly on the same day. There are also numerous dive shops in Batangas and Puerto Galera, but again, travelling more than two hours to a nearby spot turns me off so I'd rather fly an hour and be far in peace and quiet i paradise.
You can also take the course here in Manila (with pool sessions first) in Scuba World which you can do in the evenings. The checkout dive will be in Batangas on a weekend, and is normally packaged with the course fee.