British Airways is offering a small discount to AARP members, for bookings through January.
https://www.britishairways.com/en-us/offers/partners/aarp-member-benefits
The Club World discount used to be $400, so the offer is not as good as it used to be.
In Spain persons over 60 can get a Tarjeta Dorada for €6. The discount tiers are explained here:
http://www.renfe.com/EN/viajeros/tarifas/tarjeta_dorada.html
For U.K. Rail travel, one can get a Senior Railcard for £30, entitling one to 30% off most fares. However, two people can get the same discounts with a single Two Together Railcard (also £30), somthat is our choice.
In most EU countries the concession price on admission to museums and other attractions is generally for EU citizens. However, in the U.K. There does not appear to be such a restriction and we have been able to take advantage of those.
There may be Hotel chains in Europe that offer Senior discounts, but I cannot name one.
Edit: it occurred to me that Best Western might offer Senior rates in Europe, so I checked a random hotel in Barcelona. They do offer a discount of around 10% for AAA/CAA members, Best Western Rewards members, and Senior Iberia, which they say is good for "anyone over age 55" throughout Spain and Portugal.
https://www.bestwestern.com/en_US/book/barcelona/hotel-rooms/best-western-plus-hotel-alfa-aeropuerto/propertyCode.92112.html
Use the "see more rates" to check.