OK, this sounds like HDR might be a plan. At its most basic, a simple recipe would have you take 3 shots with the same exposure time, but different f stops.
Assuming that 2 stops either way will be sufficient, and figures for illustration - 20 seconds at f/5.6, 20 seconds at f/11, 20 seconds at f/22?
The idea's that one's underexposed, one's about right, and one's overexposed - theory for GIMP users (I'm assuming you like your OSS)
here (better sites are probably available).
edit: proofreading fail