Is the preposition "to" (instead of ordinary "for" and "on") possible in the sentence "When we were in the country we enjoyed going …. picnics ' And can I say "We went there to picnic" using this word in the meaning of a verb?
It is possible, but it suggests the picnics were organised by someone else, as a sort of event, and all we did was turn up: "The vicar's organising a picnic on Sunday afternoon. Can we go to it?", for example. Ordinarily, "on" is the most likely preposition.