Both feet on the same side is illegal
RCW 46.61.612: "No person shall ride a motorcycle in a position where both feet are placed on the same side of the motorcycle."
Side-saddle riding is out. It sounds like a rule written for a different century, and it partly was, but it still catches the occasional passenger who swings both legs over one side for comfort or to protect a dress or a bag.
Worth knowing if you carry passengers who are new to it. Brief them: one leg each side, feet on the pegs, from the moment they get on to the moment they get off.
You may not hang onto another vehicle
RCW 46.61.614: "No person riding upon a motorcycle shall attach himself or herself or the motorcycle to any other vehicle on a roadway."
This is the anti-skitching provision. It exists because people do it, usually to save fuel behind a truck or as a stunt, and occasionally to limp a dead bike somewhere. It is prohibited regardless of motive.
If your motorcycle dies, the answer is a tow or a van, not a strap and a friend.
The only real helmet exemption is a parade
Riders search endlessly for a way out of the helmet law. There is exactly one, and it is narrow.
RCW 46.61.613 temporarily suspends RCW 46.37.530 and RCW 46.61.610 through 46.61.612 for motorcycles operating on a closed road, during a parade or public demonstration, that has been permitted by a local jurisdiction.
All three conditions. A closed road, a permitted event, and the event being a parade or demonstration. A charity ride on open public roads does not qualify, however large it is. Note also that the suspension sweeps in the passenger, lane and foot-position rules, not just the helmet.
Renting a motorcycle: helmets are on you
RCW 46.37.535 makes it unlawful to rent a motorcycle, motor-driven cycle or moped to a person unless that person has in their possession an approved helmet, regardless of from whom the helmet is obtained.
The rental shop must be able to supply approved helmets, but the obligation lands on possession. Bring your own or take theirs; either satisfies the statute, but riding away without one does not.
Practical note for travelers: a helmet that fits you is worth packing. A shop loaner is compliant, but "compliant" and "correctly sized for your head" are not the same thing, and fit is most of what a helmet does.
Why the small rules matter
None of these will feature in a typical riding week. They matter because they are the kind of thing that shows up as an added charge on a stop that started as something else, and because two of them, the foot position rule and the rental helmet rule, are really safety rules wearing legal clothing.
The bigger point: Washington motorcycle law is spread across several chapters, and no single DOL page collects it. That is why we wrote the plain-English overview.