Проверьте пожалуйста текст, есть ли ошибки.

Не хватает глаголов:

you ARE too young for this trip.

New York, I heard it IS the excellent city,

Dear Ben
I think you should (you'd better) stay
with your parents because you ARE too young for this trip. I think you can spend
your vacation with fun, I even made A plan of activities for you until your friends () return. So you will not feel lonely - ты не будешь чувствовать...
so that you do not feel lonely - чтобы ты не чувствовал..

In June you can visit your grandmother in New York, I heard it IS AN excellent (wonderful) city, and there is a lot of things to do there (то, что у вас написано, не катит ни грамматически, ни лексически).
In July you can go camping with your parents. I think it will (could) be exciting.
In August your friends will be back (already) and you will be able to spend time with them. I hope I helped you with this complicated issue and you will enjoy your summer holidays.

Waiting for a reply!

Best wishes!

Andrey

Dear Ben
I think you should stay
with your parents, because you are too young for this trip. I think you could spend
your vacation with fun, and I even made some plan of activity for you until
your friends will return. So you will not feel lonely.
In June you could visit your grandmother in New
York, I heard it is the excellent city, and there are many entertainment there.
In July you with could go camping your parents. I
think it will be exciting.
And in August your friends will be back already
and you will be able to spend time with them. I hope I helped you with this
complex issue and you managed to spend your vacation fun and enjoyable
Waiting for a reply!
Best wishes!