。
Please come visit sometime when you're not busy.
Literal
Busy-not when [at-に] sometime please-visit.
お訪ね下さい uses the honorific お + masu-stem + ください pattern — more elevated than plain ~てください. The お-form raises the addressee's action, making this a respectful invitation. いつか (sometime) keeps the request vague and low-pressure. 忙しくないとき chains a negative adjective with a temporal noun.