1 - Buy cheap antique-like vase from thrift store.

2 - Tell girlfriend that your grandmother gave it to you and make up an elaborate and meaningful history relevant to your entire family.

3 - Wait for girlfriend to get blackout drunk. (And end up sleeping at your place)

4 – Break vase.

5 - When girlfriend wakes up, be sitting at your desk trying to glue the vase back together.

6
– Tell girlfriend that she broke the vase while she was drunk.

7
- Wait for girlfriend to say "OMG, how can I make it up to you?"