Perhaps I’m you guys’ worst nightmare, but for the past couple of years I’ve been writing haikus for my OOO, which give a flavour of what I’m out doing. A couple of examples:
I start work at 9.30am but always leave the OOO on until at least then and schedule it until 10am – that way if the backlog is terrifying, people who email me that morning will know why I’m not replying straightaway. Similar to how some people mark their first day back from a few weeks off as out of office so they don’t have meetings (which is a great idea although I rarely do it).

Easter is a time for celebration, family gatherings, and happiness. Showing everyone that you are celebrating Easter is all part of this great holiday season. You could add a few Easter eggs or bunnies to your email signature, which will make your email signature unique for the holiday season. We have many Easter holiday email signature
My OOO is almost always “I’m out of the office and will be returning on x date.” My email sig has instructions along the lines of “for questions on x, email this list” for a couple of the more common areas people would contact me about that might need an urgent reply (as the lists go to more than just me, obviously), but most people have finally figured out to contact those lists to start with, anyway. If I were in the middle of a project or something that needed to be moved along in my absence, the project teams usually know when we’re out, but I’d put a back up in that case, if needed, but generally there’s not much to be gained by a longer OOO from me.