However you feel comfortable! Sandals, shorts, and tshirt, jacket and tie, whatever puts you in the mood to have good conversations. I haven’t seen a tuxedo or prom dress yet, but hey, why not?!
Pants are generally recommended, as gender/identification appropriate.
You can also see this THATCamp flickr pool to see what has happened in the past to get a sense for it.
At early THATCamps, t-shirts and shorts were the norm. I will say that I’ve been to other THATCamps where people were dressed more formally, in what I think of as “Education Business” (cardigans, comfy but stylish shoes, etc.). That’s especially the case at THATCamps that happen in conjunction with another event. So, these days: it depends! But I’d err on the side of casual if I were you.