Usually , we use "明年夏天" to express "next summer ".

We use "下周" to express "next week ".

We use "下个月" to express "next month ".

We use "明年" to express " next year".

"周、年” can be used as the classifiers, so you can say "下周”，“明年”.

While "月" only can be used as a noun, so we usually say "下个月".

Besides , you have to consider the usage .

For example , in English , we usually say "yesterday afternoon " and "last night ".

When we say it in Chinese, we usually say “昨天下午”and “昨天晚上”。

明年夏天 is how we usually say it. But in other cases like next Sunday we say 下个星期天

明年夏天。

I think 明年夏天 is more common. :)

明年夏天 is how we usually say it. But in other cases like next Sunday we say 下个星期天