I am not a Master Gardener, but we have had MGs give presentations to our garden club even during winter. Most garden clubbers aren't fussy about what you may have to offer so winter topics might cover subjects such as composting, preparing the garden for winter and even a heads up on preparing for spring, especially pest control, fertilizing, etc... Check with local schools for what you might be able to do to volunteer in a classroom, maybe some simple garden projects for the kids. Hope this helps.

In our area, working at the agricultural fair, or gardening for many non profit groups. Local Library grounds, schools which have gardens for children, Habitat for humanity yards, answering questions at public gatherings. Look for local organizations which have a building. Ask around. Have fun! Lotta

I finiished my Master Garden classes last april.
since then, I have volunteered for the Farm Show, given a talk to a garden club twice.
I am teaching a Montessori kindergarden class once a week. There are 3 groups of about 8 or 10 Each one takes about 15 minutes as their attention span is 3 minutes tops. I couldn't find many materials at our office geared to that age so I am making my own manual. I did leaves, winter, hibernation, spring, snow, planting seeds, transplanting seedlings, foods for the pilgrims that the indians taught them to grow. pumpkins. I would be glad to send you some of my lessons if you are interested. I am running out of ideas. We have a couple of plant sales and I help with them. i haven't volunteered for the Green Line yet, but it is a good learning experience as you are always getting questions that you don't quite know the anwer to and must look it up.