If you're talking about footwear in general:
* Barefoot: This is the most obvious opposite, meaning without shoes.
* Unshod: This is a more formal way of saying barefoot.
* Sock: This is a more informal way of referring to the absence of shoes, though it doesn't cover the complete absence of foot covering.
If you're talking about a specific type of shoe:
* Slipper: While still footwear, it's a very different type from a formal shoe.
* Sneaker: Again, a different category of footwear.
* Boot: Depending on the context, a boot could be considered the opposite of a shoe.
Thinking outside the box:
* Bare: This is a broader term encompassing the absence of any covering, including shoes.
* Unprotected: This could apply to feet without shoes.
Ultimately, the best antonym for "shoe" depends on the specific context and what you're trying to emphasize.