Bonassola is a tranquil seaside town with a long sandy/pebbled beach. It gets very busy in August, full of North Italian families, but nice and quiet during the rest of the year. The beach is great for kids as well.
If you are spending several days in the Cinque Terre, come to Bonassola for a day. You can drive here or take the train, but my recommendation is to arrive by bike, on the Levanto-Bonassola-Framura bicycle path. The EBike tour is also a great way to visit Bonassola.
Bonassola is great for swimming and walking around but if you want culture, you will also find several churches and a 16th-century castle here. The most famous sight of Bonassola is probably the Madonnina della Punta, a tiny church built at the end of the promontory, to the right of the beach. It's a great panorama point, you can see all the way to the Cinque Terre to the left, and the Portofino peninsula to the right. If you get hungry, I recommend the focaccia on the lovely square close to the beach.