This is a collection of minor but useful displays that might be worth adding to an interface.
A "mouse over" is a line or two of text that describes what the mouse/pointer is currently pointing at, and which updates automatically as the player moves the pointer around. This is better for high-bandwidth interfaces, since there may be a lot of updating involved. The volume can be reduced slightly by only redisplaying when the mouse moves, or, better, when what is being looked at changes. This is probably best done by recalculating the line of text and then comparing it to what has been drawn already, although if the display is very fast, you may not save much in drawing time. One approx 40-char lines covers basic info, such as terrain type and unit type; more detail may require multiple long lines.