When you first meet Leonardo, he offers up one of your codex inventions (double blade, jump higher, poison dart) for purchase. If you have enough florins to purchase all of them or even two of them, only choose one. When the sequence is over access the DNA and replay Sequence 4 Memory 8 (An Unexpected Visitor) again. Once the sequence starts, you can hit start and skip the sequence and you will jump back to when you are offered the inventions. The first one you selected is grayed out and you are left with two options. When sequence is over do it again to get the final item.