So, of the current run of MMOs, side games can be a great extra game to play, some have good ones, some have bad ones.
For example
WoW - Pet battles
SWTOR - GSF (Galactic Starfighter)
FFXIV - Triple triad
GW2 - Super Adventure Box
for e.g. I quite enjoy both of these, but if I had to keep one, it would be the GSF, it's incredible fun once you get the hang of it, it could easily be it's own separate game.