The main 'zone' for Ranongs nightlife is the 'Walking Street' Section which has plenty of bars and restaurants. This stretch of road runs roughly north to south and is quite long. Only the last 1/3rd of the stretch has the night 'scene'.
Here is where all the farang who've opted to stay overnight in Ranong gather. A few gnarled and veteran expats run a restaurant/bar complex here which may be worth a look.
Highway 4 running along Ranong and adjacent to it has a number of Kareoke bars and a 'spa' if massage is your thing.
The other area is the Sangpla Pier region. Only come here if you really fancy scraping the barrel as it's pretty rough and ready.
The prices you pay are below average and if you're entertaining the ladies then this too is lower than elsewhere. It has to be said though, that the quality leaves much to be desired compared with elsewhere. Having said that, you do see a mix of Burmese, Thai and Laos bloodlines working the 'scene'.
Map of Ranong