I reviewed a few palm pilot games from Astraware. SOme where pretty good. But Bejeweled is always a safe bet no matter which system you are playing it on.
If it's available, get Dynomite. I don't know how it would play on your Treo, but on the PC it was an extremely addicting game. Almost addicting enough for me to cough up $20 for a licensed version because the one hour trial just does not last long enough (there are torrents for Dynomite available, but I don't like to rip off games that I like ). We played it every Friday in my microecon class. Good fun.
Anyway, Dynomite is like Bust-a-Move or Snood, but with dinosaurs... yeah... that sounds kinda gimmicky, but just trust me--it's good.