Hi Priscilla.
I knew nightlife in Sihanoukville well, but due to new developments and construction sites the whole nightlife venue has been flattened, although some bar owners started again in another part of town.
Phnom Penh is the capital and has the most vibrant and diverse nightlife.
Local Khmers go out in the weekend, local expats also tend to do so and tourists are of course around every day. It is safe to go out at night, just don't wear expensive things like a Rolex, golden chain, iPhone X or whatever. People are advised to not take notes of 100 usd with them, change them at a hotel or a supermarket, so you can pay in smaller notes.
Plenty of restaurants, not only at the Riverside but all over the city. Anything you would want to eat is available, All European and Asian dishes are available, and beyond that like Mexican, Ethiopian, even American [Hard Rock Cafe].
Coffee shops everywhere, be it the chains like Brown, Amazon or Starbucks or the independent shops offering good quality coffee.
Bars mainly in the Riverside area, from real girly bars to bars/restaurants. One of the best in my opinion is Shanghai on street 51, corner street 172. Large bar/restaurant, downstairs a pub-like area where most people eat, drink and socialize, upstairs a Blue room for life music or chosen music, very comfortable seating like a lounge, at the roof top a terrace with pool table, darts and nice view over the city.
The main girly bars are located in street 136 and 130, more in street 51 and 172.
There are two Japanese counter bars, the girls stay behind the bar, there are karaoke rooms too, they cater to Japanese and other Asian men, the girls are Vietnamese but speak Japanese beside English.
A nice way to treat your [wanna be] girlfriend is a dinner cruise on the Mekong and Tonle Sap rivers, takes about 3 hours. Before that you can enjoy a cocktail at one of the roof top bars and watch the sun set.
The ultimate jet set venue is Naga World, a huge complex with casinos, night clubs, discos, restaurants and KTV's [Karaoke upscale]. A nice place to meet hi so ladies but don't expect them to be interested in an average dressed foreigner, they have style and they are looking for style.
Transport at night is never a problem, personally I use the Pass App rickshaws [motorized 3-wheelers] and I can only advise ladies to use them too. All details about the car, the driver, pick up place and drop off are known, the cars are followed by GPS, and as you sit inside it is more safe than a tuktuk and half the price.
Life music can be found at many places too, like Tuesday night Blues at Le Bouchon near the Palace, Country and Khmer Rock at Oscar Bistro, Filipino bands at Secrets and Sharky's, Rock at Hard Rock Cafe. Too much to mention, PP is a vibrant city with lots of things to enjoy.
Joe