JaHo,
If you intend to volunteer, you might want to discuss getting a volunteer permit with the group you wish to volunteer with. Legally, you are not allowed to volunteer without a special work permit from immigration for this purpose. While this isn't necessarily strictly enforced, there have been cases when it has been and it would be wise to do some research as you wouldn't want a nasty surprise visit from immigration and the threat of deportation. Otherwise, you can stay and continue to update your tourist visa every 30 days.