About 18 km from downtown Sihanoukville is Ream National Park, which offers both boat and safari tours of its 21,000 hectares of land and marine habitat. I know what you're saying - heck, that's a lot of hectares. But it's well worth the trip with some 155 species represented including the rare mouse deer and pangolin and the very rare freshwater white dolphin, which only comes around from December through February.
As a developing resort, Sihanoukville has yet to experience a full-on retail invasion. Think of this as a good thing. It's still authentic, which means that with a bit of energy and curiosity finding something unique is still possible. By the way, in Cambodia - everything's a bargain.
With only two ATMs in all of Cambodia, and both in Phnom Penh, accessing foreign accounts means visiting a bank. Luckily, the U.S. dollar is one of two official currencies in Cambodia. The other is the Cambodian riel. Moneychangers are plentiful, shop around for good rates but inspect the bills, as even a small tear in a U.S. dollar renders it useless. Use U.S. dollars for big purchases and riels for small.