Wow, that is rather alarming that it is that easy to change variables like that. I had never used the console like that before, so I played that first game you linked to, gave myself all the lives I wanted, all the coins I needed to buy all the upgrades, whatever score I wanted... if my health got too low, I could just pause the game, set my health back to full and then continue on.
encrypting every important variable in a game seems like a lot of extra work and overhead, but what else can you do? I guess keep track of key presses, mouse clicks, and time in game so you can estimate if the score is possible (and encrypt those values too). Maybe keep multiple copies of some variables (in different forms) and test for hacking, or leave some honey-pot variables unencrypted to see if someone is trying to cheat.
Like newt said - anything can be hacked, but it should not be possible for a complete novice (like me) to completely defeat a game in a few seconds! I guess if you want to keep Global leader boards, you could keep code for testing whether the score is valid or not on the server. But they would still be able to see how the data is assembled to be sent to the server and eventually reverse engineer it.