The very first treasure I remember getting from a flea market was this pack of cards, a game called Space Race. When I got it, I was charmed by the astronomical graphics but didn't really understand the historical implications. This was part of a series of educational card games made by Edu-Cards, a division of Binney & Smith (makers of Crayola Crayons). This particular set is from 1969, although this page alludes to earlier editions.
(Click to enlarge)
The box shows a scene of an American moon landing
The first card shows the back illustration of the set. Cards 1-5
The rest of the planets (including Pluto) and the beginning of the penalty cards
My favorite card: 'SPUTNIK' - Lose Two Turns