Final answer:
The main idea is that there are several fun activities to do on a beach vacation, such as surfing, building sand castles, and playing beach volleyball.
Step-by-step explanation:
The main idea of the sentence "You can surf, build sand castles or play beach volleyball. These can be a lot of fun when on vacation at a beach." is that there are various enjoyable activities you can engage in while on a vacation at the beach.
Activities such as surfing, building sand castles, and playing beach volleyball are highlighted as ways to have fun.
The main idea of this sentence is that there are various activities that can be enjoyed on a beach during a vacation, such as surfing, building sand castles, and playing beach volleyball. These activities are described as being fun and enjoyable.