The easiest game to unlock, just complete all 5 timelines to 100% completion.

In Neo Breakout, beat level 6.

To unlock that game, you must play VCTR-SCTR until Wave 3. You can start at the Waves you unlock if you die. In the Asteroids phase of Wave 3, an enemy from Gravitar will appear. Destroy it to unlock Gravitar for the 2600.

To unlock Basic math, go in the tutorial of Haunted Houses (Graves Academy). Before the elevator, go right to the dead end, and wait in front of the door next to the ? there for a bit. You will be teleported into a class, you must now take the exam in the middle of the room and bring it to the front desk. You now have unlocked Basic Math.

Go into Quadratank in single player, and select map 2 in the Capture-The-Flag mode. You're not obligated to put bots, so don't put any, it will make it easier. Now you must collect powerups : you must max out your range and get the missiles. You must now go in the middle of the map, near the Atari logo circle, and you must control your missile to draw the circle. You will hear sound cues by doing that around it. If your circle is clean enough, the Atari logo will become a key and you'll unlock Combat Two.