I'd say that, if you're a tourist and want to do as the locals, find a beach cruiser and take a ride along the Strand between Manhattan and Hermosa. The Strand is a sidewalk that runs along the sand and connects the beach towns of the South Bay. If you don't have a bike, then take a walk. If you find a place you'd like to stop, then head down and plop yourself in the sand. I think riding/walking along the strand gives people a great feel of what living in Manhattan Beach is all about.
PRACTICAL TIP: If you don't want to look like a tourist, please be aware that the walkway that is closer to the beach is for bikes and the walkway away from the beach is for walking. Do not walk along the bike strand! You'll just get in people's way and cause problems. Also, if you're crossing the bike strand to get or from the sand... look both ways before you cross, for chrissakes!!! You don't want to be taken out by a Lance Armstrong wannabe!
The Strand, technically a street, is a wonderful pedestrian/bicycle path that goes right along the beach through town. It is divided into two parts: an upper for pedestrians and a lower one closer to the beach for bicyclists. It is very pelasant and scenic, as well as conventient, allowing one to comfortably and easily walk along the beach from one end of town to the other. In the summer, it is particularly nice since, although the narrow streets in the portion of the city west of Sepulveda Blvd are rather pedestrian friendly, they tend to be stuffy and hot day and night, with no air movement. On the Strand, one can enjoy the cool, generally gentle, ocean breezes that just don't penetrate the narrow streets even one block further from the beach.
Walk/Ride/Rollerblade down the strand (the strand is the long sidewalk along the beach) in Manhattan Beach.
The picture below was taken in the middle of December. Summer or Winter, it is nice be on the stand and watch people play volleball or surf. There are two strands, one for walking (on the left) and one for riding/rollerblading (on the right). I have noticed that visitors are sometimes confused by the two separate uses of each of the strands. Please be aware, because you don't want to get run over by someone on a bike if you are walking on the bike strand. I have seen it happen.
The easiest, and cheapest, thing to do in Manhattan Beach is to rent a beach cruiser bike and take a ride on the Strand. Ride south to Hermosa. Excellent people watching.