Yah cakes usually worth a lot on most servers, but we got fishies ^_^
Cakes still have a little worth here as some headgear quests require them.
I'd like another experience event as I didn't really have time for the last one, but I understand why you don't want to do one. I'm also being biased cause I wanna make getting to 98 on my sage less of a pain than it has been!
I like the 5% experience hat too, especially if you limit the randomization a little. Make it so it comes up with brute, fish, undead, formless, or plant only everyone trains on one or the other eventually or several at once. :p
Just a simple headgear for celebrating 100 players wouldn't make me too unhappy either, I always just like making them! If you made a quest for the hat it should be like the other custom hat quests and it shouldn't only stay up for a small amount of time (thus making it a perma quest).
Then again I just realized you could do that with the 5% exp hat too.
Maybe you can have an npc give a small set amount of exp too, but that might be a little silly >_>
I think that's all I really have to say, I don't really have any nifty suggestions to give!