Small group content that is accessible, somewhat short, has some difficulty, has some sort of objective, and does not involve grinding the same type of mob for ages.

Some examples: WOW heroic dungeons, FFXI's COP missions/ENM/BCNM/Assault/whatever, Aion's dungeons/fortress sieges/rifting/dredgion.

Solo questing, PVP, or getting achievements is also good for the days where I know I can't devote long hours to the game or where I know I can be interrupted at any time and don't want to screw over groups.