This has been me lately in DPPt. The Underground is so addicting and I love the mechanics and idea behind it. Also I spend alot of time between routes 210, Solaceon Town and 209 cause I do alot of breeding.
As for HGSS, the Pokeathlon and The Safari Zone. The games are really fun and challenging and I like the idea of customizing the Safari Zone to get what ever Pokemon you can.