In reply to Mark Kemball:
Personally I've always found swimming quite painful when I've had ankle injuries. Running and cycling will also be out obviously. It will therefore be quite difficult to do anything to maintain fitness, if by this you mean cardio fitness.
However, if what you really want to maintain is finger strength then this can very effectively be done using a fingerboard while your ankle is knackered. The obvious routines are max hangs or repeaters - I prefer the latter but try both. The advantage of both of these is that you don't need to use your injured foot at all, instead you can gently lower on to your good foot at the end of each set.
You can also train finger endurance with a fingerboard by staying on the board for longer periods, but with one foot taking some weight on a chair. I would advise against this initially as you need one foot on the chair, with the other hanging behind you ready to take your weight if your fingers fail. However it might be suitable once your ankle is improved, but not yet ready for routes or bouldering.
Fingerboarding is boring, but it is effective and you could actually return to climbing after six weeks with improved finger strength if you put the work in.
Another option, albeit much less useful from a climbing perspective, would be to get a month's membership for a gym and get one of the instructors to show you some upper body weights workouts.