For Halloween I painted my arms with Totally Tattoo temporary ink - worked pretty good when applied with a small brush. Decorating your own arms is not going to be very precise, but this worked well enough given that the Halloween party had dim light, and that the juxtaposition of me with body art is blindingly hilarious to anyone who knows me (I don't have what it takes to commit to the real thing!).
As you can see in this variegated tattoo, the overlapping scallop pattern worked best (the cross hatching, not so much). Also good: patterns with glob-y shapes, so accidental globs can be incorporated.
(me pretending that I have muscles to go with my pretend tattoos)
The fine print: I prepped the area with rubbing alcohol, as instructed, and the images lasted through the shower 2-3 days; after that, they started to look kinda ostrich-skin and worn. Removing them with rubbing alcohol was a pain in the butt, especially after they had sat for a couple days.
I got the ink from Dharma Trading, which also suggested buying a stencil tool and cutting your own stencils. Being a sucker for crafting tools, I did try that, but didn't get all that great results yet. First off, that tool gets HOT - sliced through the vinyl like a knife through butter, but I was worried that if I used it long it would ignite the paper image I put under the vinyl for tracing purposes. (I first peeled the vinyl off the backing, then put it on wax paper so I could see the image I wanted to trace underneath). When I put the stencil on my arm, it worked fine once, but afterwards gave completely blobby & smeared images. So I just went with the paintbrush for Halloween, which gives you more connected-line possibilities anyway.
P.S. With a fine-tipped little squeeze bottle, the brown ink could probably be an ok ersatz henna - not nearly as long-lasting as real mendhi, but faster-drying.