This especially works at school.

Run up from behind and cover his eyes with your hands.
Lean in, making sure he feels you against him, and whisper "Guess whooo?" in his ear.
The whisper turns him on.

When I did this, my boyfriend was like "I dunno. Maybe...
" - turned around and lifted me into a hug - "Kaylee!" It was so sweet and it made him feel manly since he was able to lift me :)