The secret code at the end translates to "Welcome to Gravity Falls".
If you look closely, you can see Blendin Blandin in the background of many episodes, including Tourist Trapped, The Legend of the Gobblewonker, Headhunters and Gideon Rises.
Agent Powers is found in the background of many episodes like Headhunters.
At the very end of the end credits, you can read "ZHOFRPH WR JUDYLWB IDOOV", which in Caesar cipher means "Welcome to Gravity Falls".
Mabel gives a boy a note saying if he will go out with her it is almost exactly like the note Dipper's voice actor Jason Ritter gives a character on his show Parenthood.