This is way too long a list. I'm admitting that in advance, and you should probably ignore it if you want a quick-and-dirty listing that fits neatly into the $500 budget. Sorry!
I think one of the big questions is whether they're more interested in game design/gaming from a theoretical standpoint, or a practical one (not saying there's anything wrong with either).
On the theory side, I would say you should definitely include some of the classic stuff from Milton Bradley (Hasbro) or Parker Brothers (Hasbro). Lots of these games are deceptively simple, and examining them would generally be a good idea in my mind (added bonus: they tend to be less expensive!):
Monopoly
Sorry
Battleship
Yahtzee
Stratego
Risk (basic)
Pit
Milles Bornes
Uno
Scrabble
Scattergories
Also on the simplicity side would be games that, on Boardgame Geek, are referred to as "Euros."
Ticket to Ride
Settlers of Catan
Carcassone
Puerto Rico
Power Grid
Then you have the other side of things, acknowledging that the likelihood of creating a game that has as wide an appeal as Monopoly that will also get people to buy it is relatively low. From there, I would say you could focus more on games that are complex enough to require niche markets, but still spur interest from a thematic or pure fun point of view. Often these games will tend to work better with house rules, which give the added bonus of getting in some critical thinking about how rulesets could be better. Lots of these will be of the "Ameritrash" variety, if you frequent BGG.
Arkham Horror
Risk Legacy (although you have to be willing for this to potentially be a recurring purchase)
Ikusa (formerly Samurai Swords or Shogun)
Diplomacy
Battletech
Axis and Allies
Fortress America
You'd also probably do well to see some of the latest games that got massive commitments on Kickstarter, like:
Zombicide
Kingdom Death: Monster
Tammany Hall
Rivet Wars
The downside to these is that you'll probably either have to pre-order them, or ask your Friendly Local Game Store to get a copy in for you. The upside is you have an official reason to go to your Friendly Local Game Store!