The game Sheldon is playing ("The Oregon Trail") was released for the Apple II in 1985 and for DOS in 1990. Sheldon has a DOS-based system.
During Missy's batting practice, an advertisement banner for "Herschel's" tire and auto repair is hanging on the outfield fence. "Herschel's" is the auto repair shop owned by the Coopers' next-door neighbor, Herschel Sparks, where Georgie got his first job and discovered his gift for tire repair.
The cross necklace that Missy wears as a good luck charm is the same necklace that Georgie wore while talking to Veronica at her locker in "A Race of Superhumans and a Letter to Alf" (s2e11).