National symbols are emotional territory almost everywhere, and casual disrespect toward them can range from social offence to actual crime. Flags top the list: in several countries, damaging or disrespecting the national flag is illegal, and even wearing it as clothing or letting it touch the ground offends people. National anthems deserve similar care; in some countries everyone stands, including in cinemas, and visitors are expected to follow.
Monuments and memorials, especially those tied to independence, war dead or revered founders, should be treated like sacred ground. Climbing on statues, striking joke poses or making critical remarks nearby is a fast way to anger locals, and images of royalty or national heroes are legally protected in a handful of countries, including on banknotes.
Traditional dress is more nuanced. Wearing local attire is usually welcomed as appreciation, particularly at weddings and festivals when invited, but treating ceremonial or religious garments as costumes can offend. When in doubt, ask a local friend whether something is honored or trivialized by an outsider wearing it; people are generally happy to guide you.