What is the difference between "我还没打算" and "我还没决定"
In the narrow sense of a q&amp;a practice, I would say that by saying ‘我还没打算’ you are not answering the question if you want／are going to travel (你暑假要去旅游吗), while 我还没决定 (I haven't made up my mind yet) is a very clean answer.

我还没打算 is like 'I didn't plan (the trip) yet'. Also, compare this answer to something like: '暑假我没有计划' (I have no plans for the summer holidays).

I hope this helps!