Single speed bikes are more popular than multispeed bikes for a reason. Most people only bike through varied terrains for the adrenaline fix, so if you prefer a casual stroll along the coast over an intense afternoon at the hiking trails, then a single-speed bike should offer a smooth ride, uninterrupted by shifting worries and derailed drive trains.
You shouldn’t put single-speed and fixed gear bikes in the same category, though. These are set up and function differently, and you’ll easily confuse one for the other if you don’t pick either out of the display racks. A single-speed bike is the obvious choice if you want to coast, but make sure you pick a gear and frame that’s built for efficiency. A fixed-gear bike doesn’t have freewheel pedals, so you’ll have to keep the momentum going; this shouldn’t be a problem if your ride doesn’t involve scenic views and varied terrains.