If you go in the party,you'll have a great time.
= If you go in the party,you'll have fun.
= If you go in the party,you'll have a good/nice/wonderful time.
= If you go in the party,you'll have a ball.
= If you go in the party,you'll enjoy yourself/yourselves.
= If you go in the party,you'll play happily.
注：/表示可以任意选用其一.
楼主是不是把go to错打成了go in?