Re: What will they do?
There are plenty of outdoor activities that can be done indoors with a little effort(or money, rather), so not being able to leave the building won't be as big a problem as it sounds at first blush. You'd likely need to shrink things like sportsfields, but we already have indoor variants of both footballs. If you bring grass seed along with UV lights, you can even have something resembling a park.
That said, at some point "self sustaining" is going to include maintaining the population without without bumming people off earth - and history has shown that boredom is an excellent population increaser.
Biting the hand that feeds IT
