You must teach them all the coolest things from your childhood: Pop Rocks; Skee-ball at CEC; Root Beer Floats; Roller Coasters; Old Monster Movies till 4am; How to Light a BBQ Grill and Burn the Hotdogs; Fireworks; Laying in the Grass at Night watching Falling Stars; Fishing; Roller Skating; Walk Down a Railroad Track; Watch Boats on a River; Go to the Tallest Building; Water Balloon Fights; and 50 other things that their parents won't ever get around to doing.
To be a good uncle, you play with them and have fun while at the same time keeping them out of trouble. When they were toddlers I would be a step or two behind my nieces/nephews and allow them to explore. At other times it would be taking them to the park or playing with them on the floor. If it is bedtime reading with them.