Depends on the vacation. A non obvious thing for a hiking trip are cotton balls. They are great for starting a fire. If you didn't forget some type of emergency fire starter...
Even if you just go on city trips you should have a sharp knife with you. I eat in quiet a few German restaurants where they gave me a blunt knife. Then I asked for a sharp knife - and they brought me another blunt knife.
Other maybe non obvious, but useful, things are charcoal tablets, ear plugs (if you have - for whatever reason - loud next-door neighbors in the hotel), a small film canister with corn starch (fast remedy for bleeding wounds), Epoxy glue (for repairing all kind of things other than wounds!) and a small, robust, powerful torchlight (there could be blackouts in any city anytime). You could also pack a sleep mask, even if you normally don't use one: some years ago I slept in a hotel room with a blinking neon light opposite my balcony. Yep, these are 'minor details' that travel brochures don't tell you...
A multitool is one of the most obvious things I pack for vacation (better check the knife laws of the country you want to travel to before). But most people I know don't even have a Swiss Army Knife.
If you are single and travel alone, always have condoms with you. You never know who you'll met and how the day will end.