I'm a keen mountain biker, so I've put my energy and frustration into developing new mountain bike trails in the hills around my house. Been meaning to do this for a long time, but there are such good trails a few miles further away, so the incentive has not been very strong until now.
I'm building for about 1 hour per day on average, and I manage to get between 10 and 100m of trail built in that hour, so by the time the lock-down ends I'm aiming to have a contiguous piece of singletrack that's a mile long.
Also, I've been helping on a local project to develop an open-source ventilator (https://www.backabuddy.co.za/champion/project/rescuevent)
And I'm working on a peer-to-peer donation platform (which is not really ready to show to anyone yet)