I hope you have all had wonderful summers!
I hope each of you had a wonderful summer!
In the first sentence, we have a plural audience (you have all). We can also assume that each person had their own form of summer vacation, independent of the others. Given that, the object should be plural, i.e., summers, not summer.
To explain further: If members of the plural audience had been served the same meal or gone on the same day trip, and then reconvened, one would say:
I hope you have all had a wonderful meal OR I hope you have all had a
wonderful day trip. [singular objects]
On the other hand, if they had all gone their own separate ways for meals or day trips, and then reconvened, one would say:
I hope you have all had wonderful meals OR I hope you have all had
wonderful day trips. [plural objects]
I have consciously refrained from rewriting the OP's example.