Just about everything, so long as you use it commercially to identify the source of your products, and so long as it is not “confusingly similar” to a competitor’s mark.
Company Name and Slogan – “Nike” and “Just Do It”
Domain Names – godaddy.com
Sounds – MGM Studio’s lion roar
Smells – Only if the smell isn’t “functional” (ie – fragrances don’t count). A trademark is registered for a plumeria scent for a company that sells plumeria scented yarn.
Product Packaging – The Coca-Cola Bottle
Restaurant Decor – Think classic Taco Bell, KFC, McDonald’s storefronts.
The key is to remember that sometimes it is not so much what is being sought for a trademark registration, but how you use it.