我个人认为a lot of fun的同义词应该是much fun.

在英语中，a lot of fun中文意思是“许多乐趣”，最好的短语是have a lot of fun,我们多用have much fun.也可以用have a good time.＝enjoy oneself.

例如：They had a good time last Sunday.

＝They enjoyed themselves last Sunday.

＝They had fun last Sunday.

希望我的回答能够尽快帮到你啊。