Pressing my lips over the button, sealing them to the belly and blowing some raspberries is always fun to do.
Using something stiff like a feather brush to twirl down inside stroking the inner walls and deep within or using q-tip or electric toothbrush is always exciting to see the reaction by either twirling the q-tip around and usin the vibrations and the bristles of the brush on the rim and then deep inside the bellybutton