You've gotten so many answers you may spend more on strings than the Flea, but I guess it's fun to experiment. I switched out the black strings on both my Flea and Fluke for Aquilas, and have seen no reason to change them, as they sound and play fine.
I think the original black strings that came on them were Hilos, at least that's what I read somewhere, and I do have some sets of Hilos that look and feel the same (I don't like them as well as Aquila nylguts, but wanted black strings for a couple of pineapples I had an artist paint).
I agree, the Flea is great for a travel uke. I'm not as fond of the Fluke because the body design makes the high frets more difficult to use, but the Flea in its denim bag has been all over, on bus tours, planes, and just seems like the logical one to bring when you may have to stuff it in overhead compartments.