My Mrs always has us in Brighton or Glenelg as well.
There is a little place called Cream, does the best coffee and breakfast. It's in no way healthy though.
Dermitt is a chain. The Gepps cross one used to be the biggest but there is one in Windsor Garden i believe that is even bigger.
They're currently building one in the old Holden factory in Elizabeth that is going to be the biggest gym in the country.
The one on Gepps cross is near what my Mrs calls a home makers centre. Has all girly homewear shops. Could probably dump the mrs there for an hour or so and get a session in.
I've never actually had the chance to go there and a totally different vibe is Powerhouse Gym in Prospect.