Domestic flights in Japan are often very good value with Avios and no surcharges. Often the Shinkansen is a better bet, but going to Hokkaido is easier with a flight in my view and to Okinawa is the best option.

Flights to/from HKG have no surcharges other than mandatory taxes/airport charges due to local law. But this doesn't seem to apply if you're transiting there, so watch for this.

Reward Flight Saver flights within Europe are quite often good value too as the surcharges can be as low as £0.50 (so under a dollar). I'm aware this isn't the optimal use but if you're very Avios rich this seems to me a good use of them.