welcome to DR1.
Custom Shirts, if there isn't already a good run shop in town, is a decent well earning biz.
bring the quality machines for the quality deep prints and stitchings and take your time to get known in your surroundings.
i changed myself very often due different reasons of stuff i didn't like,
before i hit the door of the local shop where i print my shirts now.
one keypoint for myself is, that the owner has to take his time to work my Logos with me, without asking foolish 100 bucks per logo.
i change my logos often, shirts sell in quantities per month,
he came over to me the right(by my personal preferences seeked for) way and has most likely a client for a lifetime since.
by your typing "coming back" i guess you are here now for vacation and love the local "culture", so you will leave and plan already to be back soon and open a shop, right?
as for that part in general, open a topic in the Living-Forum, you will get a ton of advise there about the theme.
good luck
Mike