Question: Given an Earth-like planet, what sort of geological features or weather systems are required for a place to have the same average day and night temperatures throughout the whole year? Is such a thing even possible?

The closest I could think of were tropical climates that are hot all year long (though even these have cooler wet seasons), but I'm looking for something more temperate, around 15-25°C during the day.

The minimum area of the region should be roughly as big as Belgium.

Answer: 
A large body of water is **part** of the answer.

The water circulation in the Pacific Basin is clockwise (bringing cold water from Alaska to California) in the summer, and *counter*-clockwise (bringing warm water from the tropics to California) in the winter. The average water temperature off San Francisco (and probably also San Diego) is *higher* in the winter than in the summer. 

This has a lot to do with the nearly constant temperature in San Diego.
This may not be the answer you are looking for, but thought I'd post it anyway: if a country is close to the equator (e.g. Sri Lanka), the temperature would be same (approx. 30 degrees C) all year around. The day time is also fairly constant i.e. sun rises at about 6am and sets at about 6pm (variation through the year may be about 1 hour).