What would be the weirdest thing that’s currently legal for the government to declare to be illegal? ~

Public displays of affection. It is illegal in Muslim countries. A sailor on one of my ships was arrested for holding hands with a female flight attendant in Bahrain in public.

Technically, the government can call anything illegal that  it wants to.   Therefore, the only weird thing would have to be the fact that it is legal for the government to declare anything illegal that it desires.

In England some people have the right to drive their herd of cattle over London bridge free of tolls. .... It would be strange to me if they made it illegal bearing in mind nobody has actually done it for 300 odd years...

Voting.

White people dancing.

In Blythe, CA, It is illegal to wear cowboy boots if you don't own two or more cows.
Between 4 p.m. and 6 p.m. it is illegal to walk a camel down Palm Canyon Drive in Palm Springs.
Flying a kite under 10 feet is illegal in Walnut.
It is illegal to drive in reverse in Palmdale.
You MUST NOT, EVER, hunt a moth under a street lamp in L.A.
In Prunedale two bathtubs cannot be installed in the same house.
Walking an elephant down the streets of San Francisco is illegal unless it is on a leash.
It's against the law to own two cats in San Jose.
In Berkley, it is illegal to whistle for a canary before 7 a.m.
It is against the law to wear high heels in Carmel city limits.
Last but not least, in San Francisco, any person classified as "ugly" may not legally walk down the street.

Meat swapping

Mayonnaise on sandwiches


EDIT: I edited my original answer "No mayonnaise on sandwiches" to maybe avoid some sort of double negative based on your question's wording; but -- what I meant to joke about was that it'd be weird, but a good thing, if the government declared mayonnaise on sandwiches illegal, ha!
:)

This post was edited by WelbyQuentin at January 6, 2020 12:53 AM MST