My two geldings, Loki (9) and Red (3) have very playful lips xD There half brothers so it's not surpising xD
I've been detangling Loki's mane and he's brought his big head over and just started fooling around with my shoulder. He pressed his teeth against me and just with -lipliplip--liplip--lipityliplip-. He's almost always relaxed and you can just mess around with his mouth, I can't help it honestly, it just cracks me up!
If you tickle Red's nostrils he'll 'smile', I often bother the heck out of him and just mess around with his mouth. And his ears...thats another thing I love. Those ears! There so... swivelish! Teehehee...Red used to hate it when I messed around with them but now he's used to it ;)