I have only been playing a short time, but have yet to figure out how I can return the favor of a couple of neighbors who have made donations of coins or motivated my production?
First you have to discover a player by scouting a province next to the players city. Then you visit the city and click on the hands under the avatar of the player. Hands symbols will be shown over buildings, that you are able to help.
click on main hall, cultural building or builders' hut (hands are shown above it, bu It already helped all my neighbors today, sorry )
If you go back to the world map a countdown (starting at 23:00h) has appeared under the cityname, meaning you can help that neighbor again after 23 hours