I have no ideas about Spa in your area, I'm afraid, but my DD and I have quite often given spa days to each other as gifts over the past few years. Just a whole or part day one usually really nice, an overnighter sounds good, too if the recipient has the time to spare.
The usual offer is one or two 'treatments', a drink, sometimes a lunch or afternoon tea, plus use of facilities, sauna, jacuzzi, swimming pool and possibly more. We love them 
It's getting more difficult to book direct but sometimes the big booking sites have good offers .
I'd ask around for recommendations if possible and take it from there. If none forthcoming, look on line. I'm sure your daughters will enjoy whatever you get for them.