[bb020708]

"次の夏休み、日本を旅行しようと思っています。"
You should add a に before次の夏休み and change the を to a へ so the sentence would be "次の夏休みに日本へ旅行したいと思っています". If you literally translate the original sentence to English, it'll make sense because in English, it makes sense if you say 'Next summer, I plan on going to Japan" but translating that into Japanese doesn't make sense unless you add a に.