Take your box of Lego and sort a number of exact similar piles. The number of piles should match the number of players. I recommend storing the pieces in separate bags for later use.
Now each player has a handful of pieces, say there are 16 in each pile for example.
1: Now the first player makes a little arrangement of 4 selected pieces from his own pile. While doing so he hides his pieces behind a standing open book or in his lap under the table.
2: When all players are ready, he shows the arrangement in the middle of the table.
3: At the moment the arrangement is shown the other players can copy the arrangement as fast as possible. But!
4: At any moment any of the players copying can say stop, and everyone has to stop building.
5: The copies are now compared to the original and minus points are given for misplaced pieces.
6: Points are noted, and the next player makes an arrangement now with 8 pieces. (repeat from 1)
Finally: The points are summarized such that the weight of points are relative to the number of pieces.
Notes: It can be troublesome to calculate points, so give and take a bit or just rate the figures quickly looking for similarity, then throw points by the paper-sissor-stone method: showing a number of fingers at them simultaneously (one figure at the time – fairplay)
-The fun part here is to just build arbitrary lego figures and train the 3D perception.