Noticed that you're already in PP - hope you're having fun.
You might want to check out Kanika Boat, which moors behind the Himawari Hotel, or Butterfly Tours. Not sure about overnight cruises, but you can definitely get dinner and a sunset!
For day tours, try Mango Cambodia who do city and countryside tours, or do a quick search on the internet - there's a few different services offered. If you're after bicycle or quad bike tours, they are available too!
At the very least, Phnom Penh rewards people who have a wander round and see what they can see. I'd recommend a walk around the Post Office Square area, and the roads around Kandal Market, for some daily life. The food market at Central Market is a good place for some lunch and people-watching too.