5. We were tired but happy because we (play) snowballs.

В данном предложении нужно использовать Past Perfect Continuous, так как действие (игра в снежки) длилось определенное время и было причиной усталости и счастья. Past Perfect Continuous образуется с помощью had been + Ving.

We were tired but happy because we (play) had been playing snowballs.

Ответ: had been playing