Actually, and this came as a shock to me, the Chinese society is traditionally a strict no-tip society. The first time I went to China, I left I think 5 kuai on the table (probably the equivalent of $0.50 at the time) for a 20Y (standard 20%) and was literally chased down the street outside the restaurant by one of the waiters telling me I'd left money at the table.

This is slowly changing, but tipping as Westerners know it is still not a practice at all, be in in restaurants, cabs, or hotel services. The candy bar idea is a great one -- you'll probably make a hit with anything that you pick up at DTW.

Most importantly, enjoy your time in Beijing!!