How should I thank someone who let me stay with them?

A very close friend of mine and his wife have allowed me to stay in their extra bedroom when I got home from active duty while I shop for and ultimately close on a home purchase. I'm about a week away from closing and I'm trying to figure out a fair way of thanking them.

I stayed with them for three months. They have never asked me for anything towards utilities and I have purchased my own food. In essence I've used a bit of water and electricity. I do the dishes when they need to be done and vacuum, my idea not theirs. I'm not opposed to giving them cash but it feels so impersonable and I want to show them how much I appreciate everything.

So...

Cash?

Nice dinner?

Gift?

Hookers and blow?

Thanks folks!