The first five minutes of the film WILL make you smile, guaranteed.
A lot of the father-daughter moments between Dracula and Mavis, on that note.
The whole scene of Jonathan and Mavis on the roof, culminating with him showing her a sunrise. Keep in mind that she's 118 years old, and has never seen the sun before in her entire life,... and that sunlight burns her up, to which Jonathan takes note of when signalling her to get behind the chimney!
Jonathan and Mavis dancing and having their first kiss.
The moment when the people of the village acknowledge that they are standing among real monsters, but they aren't afraid. They actually love it, and they even protect Dracula from the sun as he tries to find Jonathan.
Any of Drac and Johnny's bonding moments, from the game of tag on the flying tables to the scene of Drac relating his story to Jon, followed by them joking and laughing afterwards.
When Johnny admits to Mavis that he's a human and apologizes instead of rejecting him or being mad at him, she HUGS him instead. She wasn't mad at him at all for lying, because she loved Johnny so much she forgave him the second she found out.
Dracula's apology to Johnny.
Jonathan's gift to Mavis, which is a bunch of fireflies taking form of all the major sights in the world, like New York and "Ha-wee-wee" (Hawaii).