The first thing I would do is get matching water bottle cages and replace the bar tape on the Jake
Our bike fleets are very similar. I have a 2007 Novara Randonee drop bar touring bike that I use as my primary commuter, a 2007 Novara Buzz (flat bar, fully rigid, 26" wheels, disc brakes, all black) I use as a tow vehicle and sometimes as a rain bike, a 1991 Raleigh Olympian road bike converted to single speed, and a 1988 Nishiki Ariel fully rigid MTB.
I am not sure what to tell you but here are some random thoughts:
Figure out what about the Marin is bothering your knee before doing anything (I read your other thread about that)
Put better cantis on the Jake
Painting the Gitane would be a good excuse to get a air compressor and spray gun, assuming you have someplace you can paint the frame
If you are just bored you could try to build up another bike on the cheap. There is a cool "townie" thread in the fixed/SS forum that got me thinking about turning my old Nishiki MTB into a townie: replace the stem and bars with chrome ones and put some Wald paper boy baskets on the back.
Get a decent work stand and/or truing stand if you don't already have one