The new star is a Korean-style bathhouse with a large spa that includes a scrub-room, steam room, and dry sauna (separate male and female) as well as a nail salon, massage therapy, a restaurant, kids' play area, common relaxation area and communal heating huts. It's a very relaxing way to spend at least half a day.
The spa is located about 40-50 minutes from downtown (depending on traffic) so you'll want to allow at least one-half of your day if not more.
It's really worth it to spend as much time as possible so you don't have to rush.
Address: New Star Bathhouse | No.1, Lane 258, Jinhui Nan Road, near Hongquan Road | 金汇南路258弄1号，近虹泉路
Hours of Operation: Daily, 24 hours
The way it works in these bathhouses is pretty streamlined (read China Spa Etiquette here) so even if you don't speak Chinese, you'll figure it out.
You can read my blog post about my trip to New Star. Going to a bathhouse is a great way to relax after a long day or two of sightseeing or shopping. If you're with your partner or are on your own, you'll spend the spa time with the same sex but you can meet up after bathing to enjoy a meal, get a massage or hang out in the hot-house (there are four at New Star of varying degrees, the highest on my visit was 70C, that's 158F) for a while.