It's small and short, but Bill and Joe's cameo at Pebbles and Bamm-Bamm's wedding in I Yabba-Dadda-Do.
The song that plays during the Flintstones and Rubbles taking care of the twins in Hollyrock-A-Bye-Baby.
The flashback scene to when Pebbles and Bamm-Bamm were introduced to each other.
Between his tiptoeing and how terrified of his wife's wrath he is, Fred giving his family savings to the Rubbles so they could adopt a child was really heartwarming. It's also what cools off Wilma, as Fred usually isn't capable of something so kind (or that complex for that matter).