I am looking for some ideas or tips in some ground work exercises I can do with my 6yr old Paint gelding that will help develop respect and establish that I am in charge

We obviously do plenty of lunging and basic leading and backing, sidepassing etc from the ground but I am looking for specific "games" or exercises that will help our relationship develop properly. He is sweet and pretty quiet natured but does like to try and test and sometimes is disrespectful. Any ideas would be greatly appreciated!