Former Adventurer achievement requirements

You need to build all the basic units, plus all the civilization Unique Units from the original release nations (no DLC). I'm sure the UUs are required as I popped mine on a Naresuan's Elephant.

It was a surprise to me, but the achievement logging has apparently been re-enabled at some point so you can check on your progress. Here's how (Hat tip to this CivFanatics forum post):

1. Navigate to My Documents/My Games/Sid Meier's Civilization 5 (my path for Win 7)
2. Open config.ini and change the LoggingEnabled line from 0 to 1
3. Run the game and it will create a text file called "achievements_debug.log" in your Logs directory under that Civ 5 folder

Interesting note, the Spaceship parts are also tracked in this log, but I'm not sure if they're required or not for the achievement.

To get the achievement, you just have to make sure you have the postcard in your possession at the end of the game. You can set the postcard down at any time, as long as you pick it back up before you leave an area. (Otherwise some puzzles would be impossible because you need all three characters to hold a different item at the same time.)

However, for the trick you suggest (leaving it in the lobby at the beginning of the game): my guess would be that it would not be there when you return to the gift shop at the end of the game. I am going to do my second play-through soon, so I will try it and let you know.

It is not really a hard achievement if you still plan to play through the game again with other characters. It would just be a pain in the neck if you have already played through the game with all the characters because you would have to do another play-through just to get it the trophy.

Update: I tried your suggestion on my second play-through. I left the postcard on the floor of the lobby at the beginning of the game. At the end of the game it was not there still. I think you have to lug it with you.