Some people like to collect stamps that have pictures that are all of the same kind of thing. These can be a lot of fun, and can also be done without costing much. Take birds , for example. There are thousands of different stamps showing birds. Space craft are not so common, but there are still many stamps showing those.
Sports is another topic shown on the stamps of all countries.
When you want to get fancy, maybe you would like to collect stamps of just one sport, like soccer. Or, maybe just stamps of the Olympic Games. A long time ago, the mark might have been made by running a pen over the stamp. Have a look at the stamps you have. Some of the cancels might have the name of the town or city they came from.