I have watched a video about secrets of this game, so I'll share the best ones with you.
On thes ship from seal the deal, in the kitchen there is a cookie on one of the shelves. You need to crawl there and take it into your mouth
In nyakuza metro, next to the trans flag (why in the world did they put it there...) there are two windows. Througth these windows you can see some kickstarter backers.
Back to ship from seal the deal, if you find some can, kick it and then kick the other cans that spawned, you'll be awarded with cool sunglasses
On the offical website there was a refference to seal the deal which nobody has found yet!
There are some secret places where you can't get without hacks, but if you do, you'll find a cheese with "SOD OFF" written on it or near it. I don't know if that's accesable without cheats, but also on the ship from seal the deal there is also this cheese and a bottle and something else, and it all triggers a strage dialogue where cheese explodes and then bottle and something else fly into the sky
Anyway ima go look at it myself right now and also here's a tag for people who'd be interested in it @Sleepy sheep god @Lumaedra
