Once you are in Arenzano, you are spoilt for choice. Its sandy beaches are split between public beaches and different private beaches where you can hire a sun-bed or an umbrella with deck-chairs for the day. If you are quite keen in a bit of privacy and in the comfort of your own area, though, don't leave too late especially in August because you could be disappointed and find everything full.
Don't forget to try a bit of "focaccia" and if you like basil and fancy a restaurant, you must have the "trenette al pesto".
Enjoy your day!