I no longer fly BA to Japan (I prefer JAL) but paid seat selection is only important if you both really want specific seats and if the flight is likely to be full due to a Japanese holiday (Golden Week, for example) or an event (Tokyo marathon comes to mind but other events are always worth checking based on my experience). Golden Week and other Japanese public holidays mean places across Japan, bullet trains and domestic flights, get full so planning is key. If you or your wife are plane fans, Japan is a good place to experience wide bodied aircraft on short domestic routes, from the ANA 787-10, the ANA Star Wars livery 777-200, the JAL A350 on a 1 hour 20 flight to Fukuoka, a classic JAL 767 and more. Haneda airport also has a great viewing deck.

The tourist information centre as you exit arrivals in Haneda is always worth a quick visit to pick some leaflets about Tokyo and Japan, including this useful publication (vegetarian and vegan friendly restaurants and cafes) which they normally have in print there.