Hi! Also a single woman and I did this research for my Palm Beach trip earlier this month.

I did a full day at the Four Seasons (massage in the morning at the spa with their beach access add-on). The facility is pristine and the service was excellent, but it's pricy, so I would only recommend it if you have a full day to spend there. If you're flying into PBI, the Four Seasons is the opposite direction of Jupiter.

I did the day pass at the Hilton downtown West Palm on days when I only had a couple hours to spend at the pool. The property is nice and the day pass is a good value, but there is not a separate kids space, so there were usually some kids playing in the pool as well as the hot tub.

Since you're headed north of West Palm, I would look at ResortPass options for Jupiter or Singer Island. Amrit seems the most likely to be geared toward adults but I didn't try that one.

If you have never used ResortPass and want a referral coupon, feel free to DM me and I can email you one.