If you have bucket seats... there are 4 bolts on the sides that can be loosened and if you slide the whole thing back or forward you can gain about 1.5" in height. I'm also 5'6", I swapped out for aftermarket bucket seats and was pleasantly surprised that I gained two inches, made a world of difference. I can still loosen up the side bolts on the original seat track frames to get more height, which once I get to offroading will probably happen because you need as much visibility to your tires as you can get.
You can also get spacers, you have about 1.5" of extra thread on the seat tracks poking through the bottom of the car. Besure that when you are done, that you still have a solidly mounted seat and that it doesn't wobble from side to side or front to back.
Thinking more about this the seat tracks are bolted to the seat frame... you won't get much height this way, as the rear seat track has stud that I think only has 1/2" extra thread on it, and the front may the same stud or it might have short bolts (mine had bolts), it would be easy to replace the front bolts, but the limitation is the back stud.
You could up the height by as much as 3.5" inches if you wanted to, and then after that its phone book, reupholster with thicker foam, or something truly drastic (and costly)
Hopefully this helps some.