In Palau, there isn't a lot of night life other than having a drink at Kramers, The Drop Off, Rip-Tide, etc. For the day, there is snorkeling off the beach at Palau Pacific Resort, which is safe, and Kayaking around the Islands. There's a Beach at the Rip-Tide, and Aquariam, etc. But not a lot else.
You may want to consider staying on Peleliu at the Dolphin Bay Resort. There will be zero night life there, but there is readily available snorkeling and it's far closer to the good dive sites (Blue Corner, etc.). Because of the short boat ride you can beat the boats out of Koror to them in the morning. But be advised, there's not a lot going on out there, so you could get bored. You'll be fairly isolated if you stay and dive with them, but I've heard good things.
If you have the money, the safe bet would be to stay at Palau Pacific Resort on Koror. It's very nice, and there are things to do for your girlfriend. But it's not cheap. The dive shop there, Splash, caters mostly to Japanese - so you will probably want to book your dives through Sam's Tours, Neco's, or Keith Santillano at Dive Palau if you want your DM speaking english. They will pick you up from the PPR. The PPR is probably the nicest place to stay in Palau, but they charge for it.
For considerably less money, you can stay at the Waterfront Villa of Palau. It's clean, and there's an enclave of ex-pat American's who live there full time in the condo's and have a cookout most Friday nights. The bar, Capn' Fins will have a few people in it at night, and the food is great, (but slow).
