Reading on May and Going to

(TESOL Worksheets--Future Forms, Going to) 

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[For Young Learners.  A reading on "may" and "going to".]

A trip to the beach.

I'm going to go to the beach with my family this weekend.  We're going to drive down there in my father's car.  When we get to the beach, my brother and I are going to build a sand castle on the beach.  I am going to bring my swim suit.  If the weather is hot, I may go swimming.  If the weather is cool, I may take a walk along the beach.  We are going to eat dinner at a famous restaurant near the beach.  I haven't decided what I'm going to order yet, but I may get octopus, since the beach is famous for seafood.  Or I may get pizza, because I really like pizza a lot.  Or I may just skip dinner and get a big piece of chocolate cake instead––if my dad lets me.  He may not allow it, because he can be quite strict about sweet foods. After we spend all day on the beach, we're going to drive back.  I may sleep on the car ride home.  Or, if I'm not tired, my family and I may sing songs all the way home.
What is he going to do?	What may he do?

What is the difference between "may" and "going to" ?