It's not cheap, but the spa at the Grand Wailea actually can be a really good value and is INCREDIBLE. It's not going to fit into your $100 category (and I doubt anything other than the schools would buy you more than an hour or so -- even if you weren't in one of the most expensive places anywhere) but in addition to most of the services, the spa treatments at the grand wailea include 1 hour of terme water therapy, which are all of these really neat water treatments you can enjoy at your leisure. So you could book an hour long massage for $155 plus you get the additional hour of terme treatment, which is one of the most interesting spa features I've ever seen.

Because these involve various baths and hottubs in an open room, this is a great group activity. One caveat: most women go in the buff through the terme. You can wear a swimsuit (and according to my husband, most of the American men in the men's terme do), but if that's something these women aren't comfortable with, it may not be the best option.