This is a great question, because it allows a lot of creativity on your part. You could honestly pick anything to be symbolic of Romeo. Your main job is defend why your symbol is appropriate.
Since Romeo is in love throughout the entire play, I think an appropriate symbol for him is something that is symbolic of love. You could choose a regular, nothing fancy, Valentine's Day heart. It is so symbolic of love to the point that the heart shape is incorporated into jewelry and candy. The internet is even loaded with people making heart shapes with their fingers and thumbs.
I think another appropriate symbol for Romeo is a rose. You could use a rose for all of the same reasons that you would use a heart. A rose, especially a red rose, is symbolic of love. That's why roses are given on Valentine's Day and anniversaries. A rose is also a good choice because Juliet directly refers to Romeo as a rose when she says the following:
A rose by any other name would smell as sweet
A third symbol that I associate with Romeo would be a skull and crossbones. No, I don't think Romeo is a pirate. A skull and crossbones is a common symbol on known poisons. That is how Romeo chooses to end his life when he believes that Juliet is dead. He drinks poison.
I even think a cross is a good symbol for Romeo. I absolutely do not think he is a Christ figure. That's not why I associate the cross with him. I associate the cross with Romeo, because it appears that he (a Montague) is more religious than the Capulets. The reason that I think this is because of the amount of time that he spends with Friar Lawrence.