改天咱们一起吃个饭吧。
i think the above one should be a good answer to your question, it's not literally translated though. but as a native speaker, i think it sounds rather natural and convey what you were trying to say rather well. ”someday“ should be translated as 改天 in my opinion;p
You could also say like this :下次找个时间一起吃个饭吧。it's english translation version should be more like:we should go get something to eat SOMETIME. ;p
Anyway different languages get into different ways of saying things, you'd better figure that out as you go along, so do i. ;p Hope it helps.