Fast Food is one of my 8-year-old son's favorites. (No doubt heavily influenced by the "You're getting fatter!" messages and the Burp at the end of the game.) My wife only likes to play a handful of 2600 titles and Fast Food is on that list. I think it's a good approachable game.
I invariably think of Spacemaster X-7. It's just rare enough that it's not going to be well known outside of the more die hard 2600 fans. It's fast, addicting game play. The difficulty level ramps up pretty evenly so you're always tempted to try to do a little better to beat your last score.
Midnight Magic may not be obscure enough to qualify as a hidden gem, but it's a fantastic pinball effort for the 2600. I find myself returning to it frequently when I'm not in the mood for the usual "the enemies multiply and speed up until you die" style of play.
My wife used to buy me the occasional 2600 game she might find in an antique store. That was easier before I already had most of the common games. Of course upon unwrapping the cartridges there were always the inevitable questions of where did you find these? And, were there any more?
A few years back at Christmas she got me the set of Atari pint glasses they sold at Target.
She's usually not that crazy about hitting up the fleamarkets with me and her attitude toward my gaming stuff is more along the lines of resignation than support. That's why I was surprised when she readily agreed to go with me one Saturday and didn't even say anything silly like "Are you sure you need that?" when I bought a nice Intellivision system with a batch of games. In fact, I was kind of on the fence about it and she actually told me I should buy it. Later that evening she told me we were going to have our first baby. It was a planned pregnancy so it wasn't completely out of the blue and of course we were super excited. It turns out she was being extra agreeable because she wanted to give me a day of carefree fun before everything shifted into baby mode. Not exactly a gift but this thread made me think of it. (And as weird as it is, I now associate the Intellivision with finding out I was going to be a father.)