Can it, Poindexter!
A "poindexter" is someone who looks and acts like a nerd, but is not smart enough to be one. So what Grunkle Stan is saying is that the author doesn't know what he's talking about. This could explain why the apocalypse predicted by the author did not happen.
Journal 1 pages
Unicorn
Magic hair
Floating cliffs
A cryptogram on that page (symbol substitution cipher): "My compass goes haywire the closer I get to them does this mean what I think it does: the answer may be underground." I don't know about this, but some people believe it refers to UFOs. And according to DaMGtMaNF, "Something strange is buried under Gravity Falls"
Alchemy page
An image on that page matches the image Wendy is looking at in the theme song.
Feast your eyes!
The labels on the fireworks include Smokey Joker, Cop Callers, Poor Choices, The Heart Attack, BOOM!, and The Lawsuit Maker. Also, getting yourself and your brother up at 7 am the night after a huge party. Smooth move, Mabel.
Dipper's water balloon
Dipper throws a water balloon at the ground and it doesn't break. Odd, considering that it breaks later due to gravity alone. Also, Mabel makes cleaning up fun!
It's unnatural for siblings to get along as well as you do.
I didn't think about this at first, but Grunkle Stan could be hinting that his own relationship with his sibling(s) may not be that great.
We've still got plenty of summer left to drive each other crazy!
The same thing was done in Gideon Rises. The writers wanted to make sure we knew that, even though this was a half-season finale and the show was going on hiatus, the summer isn't over just yet. Now, I've come across information saying that this episode was originally meant to be the season 2 finale, and the rest of the season was going to be season 3. However, since the internet figured out who the author was well in advance, and Disney told them they'd be going on hiatus in the middle of the season, they decided to reveal the author early. A third season is still possible, but not green lighted yet.
What did I do that warrants... this much arresting?
Northwest Mansion Mystery pt 1
