One thing to consider -- for gravel paths, tire width is probably more important than wheel diameter. In other words, a fat 20" would be better suited to your riding than a skinny 24".

Of course, you could also get a fat 24".

With that in mind, take a look at the Xootr swift. It will fit in the trunk of most mid-size sedans no problem, and I bet it would fit in the rear of Wrangler. Put on some wide cushy tires, and it's a great riding bike.