Something for 4-6 hours. Close to downtown preferably.

Renew day spa in Marietta is my absolute favorite. It's a female only staff, very quiet and relaxing environment. They give a very yummy massage and easily adjust to your pressure preference. The facials are wonderful and include a shoulder or foot massage. My favorite is the skin glow body polish, you spend about 30 minutes under running water while they scrub your entire body with scented bath salts, they then moisturize your body and give you a light massage, it's really lovely. The prices are very reasonable, particularly for extended hours. I've been more times than I can count. I'll link to them below.

... at Jeju Sauna in Duluth.  Admission includes all the different saunas there, and space to relax.  Other services are extra.

I whole heartedly agree on jeju.  It is co-Ed but seperate areas for men & women.  Be prepared, the wet area(hot tubs, steam, etc..) no clothing allowed.  There is nothing questionable about the place and an awesome way to spend the day! Remember to bring head phones and music.