Etiquette/social norms for a grad student toward a faculty going on sabbatical
When a professor goes on sabbatical for a year they know full well that students will graduate in the meantime. They have seen this happen from both sides (they were students once and their professors took sabbaticals as well.) There is no standard way to say good-bye. A note,  either electronic or physical, delivered before you leave is appropriate. So is asking the professor in question if they will be attending any of the graduation/end of year ceremonies that you will be having. Or you can simply say good-bye the last time you are going to see them before they leave since it may be the last time you see them. 

General advice for keeping it from being "really weird" is to keep whatever you do short. Rambling will make it get uncomfortable for both of you. You have three basic things you might want to say: thank you, good-bye, best wishes for your sabbatical. You don't need a lot of words to get through those ideas.