How about a fun mani? I just finished this and had to share it with you.
Clear nail polish
First, flatten out the tinfoil as best you can. It doesn't have to be perfect, but get all the big wrinkles out.
Lay the tinfoil on your nail and press down to make an imprint of it, and then following the lines, cut out the shape.
Put a layer of clear nail polish on your nail, and press the tinfoil down, making sure to press it around the edges.
If you're using uncolored foil or one without a pattern you could add a very quick topcoat but take note that the colors will melt if you go over it too much. That might make it look cool too, though! When you're done, just peel the tinfoil off and use nail polish remover for the clear polish. If you want this to hold better, I would suggest trying nail glue, but I have none to see if it would work.
If you do this I would love to see your version!