A class might be fun, too. Community centers and continuing education departments have every class under the sun. I think that would be less stressful than volunteering because you can choose your level of engagement. If your social anxiety is such that just being out of the house and in the same room as strangers is already a win, then you can hang out in the corner and just absorb information. Once you're comfortable in the room, you can push yourself to be more gregarious--maybe talk to the person next to you, volunteer to answer a question.