I get checked in at certain places although I am just passing by. How can I change that?

You can change each place's radius. If you minimize it, you must be closer to the place to check in there.

Android: To change a place's radius, tap on the place within the places tab, then tap the pencil in the top right-hand corner. Now you can minimize the radius at the bottom of the screen and confirm the edit by tapping "done".

iOS: To change a place's radius, tap on the place within the places tab, then tap "edit" in the top right-hand corner. Now you can minimize the radius at the bottom of the screen and confirm the editing by tapping "save".