There is one restaurent which specializes in congee on Gerrard. I know it as "Tak Kee" but don't know the actual English name. It's past the hub of all the stores, near Howland on the north side of the street. If it's not there anymore, I guess most of the other restaurents would serve it (especially the ones that serve dim sum).
As to what to order, here are some suggestions: meat balls (pork); fish; chicken with black mushrooms; salted pork with thousand-year egg. The "seafood" one may have reconstitured dried cuttlefish which may be a little strong.
Usually one would order a fried pastry to go with it, e.g. the simple flour sticks "yau tiu". Or a steamed pastry with meat stuffing "cheung fun", or fried noodles.