Flyball is definitely the greatest - don't worry if your dog doesn't pick it up immediately, some take longer than others. However if you can start getting your dog to retrieve a ball (or he/she already does) then you're part way there.
My little Ellie who was completely ball mad took over a year to figure it out, but it was well worth the effort and wait because she is a natural now and has surpassed her big sister Misha & big brother Roly who both picked it up very quickly - she is faster and has a much better technique.
The dogs absolutely love it