It's really a good idea___(go)for a picnic on weekends.急

It's really a good idea___(go)for a picnic on weekends.急
It's really a good idea___(go)for a picnic on weekends.
急

It's really a good idea___(go)for a picnic on weekends.急
It's really a good idea to go for a picnic on weekends.

应该是：
to go
it's a good idea to do sth 固定句型，
做某事是个好主意
后跟动词不定式　to do sth
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