How about a Nintendo DS?
And your being really unfair to the Wii there, and it is HDTV compatable, its faster than a xbox, and its more than a gimick (everyone thought the DS was a gimick, but we know how that ended up). The Wii has many "serious" games, just because developer like making cutsie games for the Wii doesnt mean theres no series games at all.
I'd go with Wii or DS if you want more casual.