You can't go wrong with a few different ones especially if they're inexpensive. I have a fairly nice 10X but it's small, so it doesn't always work well with my old eyes. I also have a cheap 40X that was $7 at an antique store. Larger optics, so I use it more. It has a light which failed after about a month, and the kids have been rather unkind to it.
I also have one of the USB microscopes, which can get to pretty decent magnification I think 450 or so, maybe more.
I also sometimes use the scanner and scan at 1200DPI, not a portable solution, but the best viewing of the entire card.
I also have access to an electron microscope. Unfortunately, it only accepts stuff up to about 3/8 inch diameter. But it will go to around 27000X
Oddly, I don't usually take a magnifier or anything to shows when I go. Years of looking at real stuff makes the stuff that looks "wrong" really stand out.
Steve B