Dog poop scooping? Personal couriers? Game testing? Something we haven't thought of because the relevant industry doesn't exist yet?
Economies aren't static. You can always find something useful to do.
Today I've seen adults in their 30's and 40's jump out of the IT industry to start their own business scooping dog poop out of back yards and making enough money to live on. Same thing with cleaning swimming pools. There are lots of services they could provide.
It's a little disappointing that despite immense advances in technology and production, and increased efficiency, all we're achieving is having people toil the same amount as before, but doing something else.
Most people seem to be choosing to spend productivity gains on "more stuff" rather than "more time". And that is certainly their prerogative. :) I mean, with so many people working so productively for so many hours, there is a lot of cool stuff available nowadays.