I've been on several retreats, at four different places.
Things I like to find: comfortable chairs, plenty of workspace - 2 tables in an "L" per person is fantastic if you have the space, lots of outlets - power strips on top of the tables so that people don't have to crawl on the floor to plug things in, plenty of design walls, big cutting surface at correct height, cutting mats (if you provide any) in decent shape (no huge grooves, etc), large ironing boards, fast and reliable wi-fi (surprising how difficult it is to get this one right!), high-capacity ice maker, fridge and microwave available to campers, sitting area or lounge separate from sewing room for getting away from the machine - bonus for tv and either chick-flick DVDs or Amazon Prime stick or Apple tv or similar, comfortable beds far enough from sewing room that late night or early morning sewers won't disturb sleepers.
I'm sure that I've forgotten things, but that covers most of it. I don't require much entertainment - I'm there to put the pedal to the metal and sew!