how can I spruce up boring wall tile in the bathroom
If you don't want to redo the tile you could stencil it. Here is a tutorial I found on it.
They also sell waterproof tile stickers that are just placed on top of the current tile. You can find them on Amazon.
You can buy tile decals ....they stick on the tiles and can be removed. Buy some whimsical ones and apply randomly for an interesting and fun effect
If you don't want to paint, you could replace with grout with an accent color or glittery grout.