What audrey said earlier, retirement is the treat! What more do you need than glorious freedom!
I guess I figure I got to ER by not spending money for "just because" occasions, why start then? And I didn't deprive myself of everything to get to ER, so no need to wait for retirement for reasonable things you want. And I didn't want to start ER with a standard of doing anything too extravagant or creating excuses to spend money. But if you're going to start a hobby with your new free time and need equipment, that makes sense. A new car can make sense if your needs change, such as swapping a commuter car for an expedition car.
I think a nice trip is a good idea, a way to decompress, if that does it for you. I took a ski trip, but I take a ski trip most years so that wasn't too special. A nice dinner out is another way to mark the occasion and take time to reflect, and start planning and dreaming for the future.