I say don't rush it.
Our son started showing similar interest around 22 months old. We switched to pull-ups so that we could do the up & down "like undies" thing. We too went through an excited phase and then a not so interested phase. We just stayed persistant, but didn't push it.
About a month ago (son = 29 months old) we bought some "fun" undies and told him he could only wear them if he used the potty. Since then we have had MINIMAL accidents and he even does great at daycare all day. Honestly, it was the EASIEST thing to do. We still use pull-ups at night --- he is not night trained.
*Note -- like many people, when we made the switch to undies we didn't go back and forth with the pull-ups. At first I felt that maybe we were shooting ourselves in the foot with the pull-ups, but now I can see they were a useful stepping stone for us. You just can't expect the kids to "stay dry" in the pull-ups like undies -- it is different.
We do give "potty treats" -- 2 jelly beans -- and although they used to be an everytime occurance, now they are only when he remembers.
Good luck.