The obvious day trips from Sihanoukville are the offshore islands — Koh Rong and Koh Rong Sanloem — and Ream National Park to the east, both covered in things to do.
Further afield, Kampot (about 1.5 hours east) is a sleepy colonial-era riverside town that has become one of Cambodia's most likeable destinations. Kep just beyond is famous for its crab market and the pepper plantations on the surrounding hills. Both work as overnight stops rather than rushed day trips, but either is feasible if you start early.
The Bokor Mountain plateau above Kampot — with its ghostly French hill station ruins — is a long but rewarding day excursion.