I think this phrase means, you will get great fun from doing something you are going to do.

I (we) know of something fun that we should do OR We should be having fun I would usually expect to hear such a statement if someone had planned a party and wanted to get it started. This would be especially true of some sort of fun activity followed a period of hard work. Then the message would be "Let's have fun! We have worked enough!"

It's a play on the phrase "there is work to be done". The meaning is reversed by replacing work with fun.