The next day I decided for once I'd have_________with a test.

明天我决定进行一次开心的测试（测验）。
语法讲解：
have fun doing sth（ 做某事有乐趣）. [举例]We have fun learning Enlish.(我们学英语有乐趣)

have fun with sth/sb:和...玩的很开心.[举例]He had fun with his friends.(他和朋友们玩的很开心.)

一楼回答的不错，的确是的。