In the UK a genuine "thank-you" would be plenty.
The rest of Western Europe has less of a tip culture than the UK so a merci, danke, gracias, grazie etc should be equally well received.
You tip, they'll think 'typical American chucking their money about' but happily take it. You say "thank you", they'll think 'nice person' and go about the rest of their day feeling they're making a small difference in the world.
Enjoy your trip.