It's one, two, three casinos in one
Bally's Atlantic City Hotel and Casino is one of the four AC casinos owned by the Harrah's company (along with Caesars, Showboat and Harrah's). It was originally known as Bally's Park Place. (If you're familiar with the old Monopoly game, you'll remember Park Place as the top-of-the-line property on the board.) Hotel rooms at Bally's are split across what are essentially three hotels: the main tower (with it's changing lights at night, one of the city's primary landmarks), the Claridge Tower (what used to be the separate Claridge Hotel and Casino) and the Dennis Tower (in the Dennis Hotel building dating back to the days pre-gambling, it has a 19th century decor to match the "Wild Wild West Casino" downstairs).
And if the three Bally's casinos aren't enough, the Caesars Atlantic City Hotel and Casino is right next door and connected by an indoor walkway. That also means you won't have to walk outdoors to get to the spectacular new "ThePier Shops at Caesars".
There's still a big spa at Bally's, not surprising considering it used to be part of the same company as the Bally's fitness centers. The spa has seven whirpools and offers a wide variety of massages and "herbal treatments". The spa is on Bally's 8th floor, where you'll also find a nice indoor pool in an atrium setting under skylights. In addition to the fitness equipment in the spa, there's a second fitness center in the Claridge portion of the hotel. The Claridge fitness area includes a 20' by 40' indoor pool.