Anachronism Stew: In one of her letters, Leslie tells Albert that she's the captain of the girls' basketball team at her school, despite this episode taking place about a decade before the sport was invented, in 1891.
Career-Ending Injury: Leslie tells Albert that she's captain of the girls' basketball team and a ballet dancer at her school. It turns out that she was indeed this before being paralyzed by an accident.
Dude, Not Funny!: In-universe; Willie gets sent to the corner when he's asked to define what a pen pal is (someone who lives in a livestock pen).
Hiding the Handicap: Leslie hides the fact that she's weelchair-bound in her letters to Albert, instead sharing facts from when she could still walk.
Honesty Aesop: At the end, Albert and Leslie meet for real and decide to go on writing, but only telling the truth about themselves.
The Münchausen: Albert, who makes up that he's incredibly tall, athletic and once saved his sister from an uncontrolled horse in order to impress his pen pal Leslie. Meanwhile, Leslie technically doesn't lie about her doings and traits, but they're all from before she was paralyzed in an accident, a condition she hides in her letters.
