I hope we ------(have) a good time in the park.

have
此句的语法为I hope +that 从句,实际语句为I hope （that）we have a good time in the park.
只是that省略了.
且：英语句子前后语法时态保持一致.我希望我们在公园会玩的愉快,动作还没有发生,且前面为hope,并不是will hope,所以后面用have,不用will have.