When you do and do not need a costume
As a guide, I would rather see you comfortable and prepared than wearing an outfit that causes problems later in the night. Check the final venue’s dress code, especially if your costume includes shorts, a tank top, flip-flops, a large prop or a face covering. At the final club on my pub crawl, flip-flops, shorts and tank tops are not accepted. Staff may also need to see your face and check your physical ID.
For my Halloween pub crawl, a costume is not a substitute for the practical details: guests must be 18 or older, bring valid physical ID and arrive at the meeting point shown on their booking. If the event page gives a specific dress instruction, follow that instruction.
A few details make a difference
You can absolutely enjoy the night in ordinary clothes. If dressing up sounds fun, keep the outfit easy to walk in and suitable for every venue on the route. A good costume should add to your evening, not make the door policy or the journey home harder.
If you are joining an organised event, read the dress information even when it is advertised as a costume party. “Halloween theme” does not mean every venue accepts every outfit. The final venue may have a different policy from the first bar, and staff decide entry at the door. If the rules are unclear, describe your outfit to the organiser before the night rather than discovering an issue when the group arrives.
Choose what suits you
There is no need to buy a new costume just to participate. A themed accessory or simple makeup can be enough to feel part of the night. Choose something you can move in and stay comfortable wearing for several hours. Whether you dress up or not, bring valid physical ID and make sure staff can verify your face when needed.