It depends on which version of the game you buy. Highskill64 has listed the digital download benefits. if you want a physical copy currently there are these options

Brawler steelbook - if you live in the USA a random steelbook of 3 designs

Clouds nail bat weapon - Amazon pre-order (you can pre-order any version incl digital deluxe to get this)

There's also the UCE no additional benefits other than being entered into a prize draw to win a bunch of stuff but it's the most expensive as it has things like the season pass, artbook, a wol bust etc

If you live in EU you can buy the UCE or the standard edition. Not a lot on offer for pre-ordering though.

special items pack - Amazon are apparently offering nameless warrior, Garlands Claymore weapon and a secret item (in the bonus details)

steelbook - not sure if this is the design or a place holder image though.

That's all I know of so far. If you're outside of US or EU I'm unsure what else is being offered. The nail bat is new so other places may start offering some pre-order bonuses soon.