After tiptoeing around it for the first few volumes, Calvin's family starts to let their problems with each other reach the forefront. There are a lot more exclamation points and a lot more smart remarks from all members of the family, particularly Calvin's parents. They behave more like they're sick of their son's antics and less like these are new experiences.
Four strips/stories stand out in my mind from this book as particularly good. One is Calvin's Dad's plans for the Christmas tree, where he fabricates the idea of leaving it in the garage, saying Calvin can take a look at it "if" he gets "a" present. Calvin's reactions are priceless on their own, but the consequences Calvin's Dad faces courtesy of his wife along with his final quip put it over the top. It's not much of a left turn, but it's a strong sign that the strip is maturing.
Another of my favourites is the family camping trip, easily the best story in the series' history up to that point. It starts to rain during the paddle in to their campsite and doesn't stop. There are so many great moments from Hobbes drying off inside Calvin's tent, to the father quickly having the "At least it's not snowing, right?" line thrown back in his face, to Calvin wringing out a towel after going swimming so he can dry himself off before being instantly soaked. It's brilliant and hilarious, punctuated by the family's ongoing competition of sarcasm.
The harshness between the family makes the softer moments that much sweeter. When Calvin attempts to secede from his parents and move to the Yukon, the failed effort results in his Mom hugging him and telling Calvin he'll always be her kid. No sooner than that happens do we realize that Calvin has left Hobbes in the woods. After sunset, Calvin's Mom is devastated and sends her husband out for a search. While Calvin's Dad begrudgingly does so, it's a beautiful moment of great parenting.
But this strip isn't about mushiness, which leads me to one last story. This collection has the first truly great "Spaceman Spiff" fantasy and maybe the best ever. Trapped on a barren wasteland of a planet, Spiff peers over the horizon to see a giant, nasty creature... offer him a sandwich and lemonade. The drawing of the alien with a tray of goodies is too funny to describe.
We're still not at the height of the series, but "Yukon Ho!" is the first "Calvin and Hobbes" collection that you can't miss.