Collecting Guatemala

I am interested in getting member thoughts on areas of twentieth century Guatemalan philately which are interesting and affordable. Not all of us can afford to collect 19th century covers. There are a lot of twentieth century areas of philatelic interest which are affordable and can bring a lot of collecting satisfaction. The 1930's internal airmail covers are easily found for less than ten dollars, but are interesting because of the flight schedules, and limited number of interior destinations. The postal 'stickers' of the late 1990's are all internal usages but despite difficulty finding them they are usually very affordable. Nocturno covers beginning in the 1930's are another unusual topic which is relatively unknown and an interesting topic.
Do you have suggestions of collecting areas that would be interesting to collect?