What should i get engraved on his flask?

It's my boyfriend and I's one year anniversary and I wanted to get him something he'd really like and he's told me he would love to have a flask just like his best friend. So I decided to get him one but I want it engraved so it's special but I have no idea what I want on it. I want something that is special to us but is still cool to have on his flask. Any ideas?