gave Susie a mushy valentine

Nope. Try again.

In their reality, it was duplicate of Calvin who did that. Interestingly, it didn't happen in Susie's reality either: Calvin gave her a sarcastic valentine with the same words. See Attack of the Deranged Mutant Killer Monster Snow Goons (also reprinted in Calvin and Hobbes Tenth Anniversary ).
Artwork © Watterson/UPS; click for notice.