Two kinds of water
Arima is unusual for having two distinct types of spring water in the same small town: kinsen ('gold spring'), a reddish-brown water rich in iron and salt, and ginsen ('silver spring'), a clear water carrying radium and carbonate. Several bathhouses in town offer both, and trying each is the main point of a visit.
Getting there
The most straightforward route from Osaka runs via Kobe — a train to Shin-Kobe or Kobe Sannomiya, then a connecting bus or the Kobe Electric Railway up into the hills, with a total journey of roughly 60-90 minutes depending on the connection. It's a longer haul than Osaka's other day trips, which is why an overnight is worth considering if your schedule allows it.
Day-trip vs overnight
Several ryokan in Arima offer day-use bathing packages specifically for visitors who aren't staying the night, which is the realistic way to fit the town into a single day from Osaka. If you can spare a night, though, this is one of the closer, easiest places in Kansai to try a proper traditional ryokan stay with in-house hot spring bathing.