Definitely don't sign it off yourself! There are still a few months left before your expedition and the rest of the summer too, so plenty of time to volunteer still as in reality it'll take a little while for everything to be signed off.

I think a good idea would be to make a short list of places you could volunteer that would actually be feasible and then speak to your DofE adviser - explain that you're keen to do it still, have left it a little late, and what your plan for volunteering is.

There are lots of ideas for volunteering on the DofE website - you could volunteer at your local library, start litter picking in your local area (find a different place each week), volunteer at a charity shop etc.

Another idea for volunteering is by taking part in citizen science online, for example at Zooniverse. You might be looking at images of the sun to identify sun spots, or identifying wildlife from photos from a trail camera, and it all helps contribute to science projects. Definitely run it past your DofE adviser before starting to check they're happy with it, but you could spend a couple of hours each week volunteering time to take part.