It is very common for people to add gifts, or relevant goods, to the coffin or grave of a loved one. You can read here https://www.funeralzone.com/blog/famous-graves-goods-celebrities about some of the things some of the Hollywood great and good have been buried with.
This custom goes back centuries, with the Egyptians taking it to the almost extreme. My Grandfather was buried with his darts, just in case he wanted a game. I carried the coffin of a close friend who's family had added lots of his favourite tools (in case he needed them), it made for a very heavy coffin!
As a Celebrant it is common to be asked to place something on the coffin during the Committal (the part where you say your final Goodbye). I'd like a glass of champagne placed on mine, whilst everyone else toasts me on my way with theirs.
I often place a glass of something alcoholic or some sweets on a coffin. I once placed an empty coffee tin as the son-in-law had always threatened to tow the ashes behind his motorbike in a coffee tin! There really is no end to what can be added at the Final Farewell. Sometimes I close a photo that has been placed on there as this can signify the ending.
There are, of course, things you can't add for safety reasons if the body is being cremated. Your Funeral Director can guide you on this. If the body is being buried, or the ashes interred, then items can be added, maybe as an alternative to earth.
Writing this has got me thinking about what I might like added to my coffin to see me on my way. My favourite teddy who sits on my bed (more so that the family don't have to deal with what to do with it after I've gone), some crisps (I love crisps) and maybe a Milky bar (white chocolate is my favourite). I have a fifty/fifty conflict here as I believe that once you have gone, you have gone so it is pointless adding things to take with you, but I also like the idea of 'packing for a trip'!
I hope that this starts a conversation with your family and friends about what you would like/would hate when the time comes. It's an interesting conversation piece, that's for sure!