While there have been some great suggestions, the carriage ride one screams proposal, your son should avoid ANYTHING that could say that to her! My SIL had a few fake outs that had her very upset before her now husband actually did propose. I always warn my 19 yo son to skip little ring-shaped boxes! :)
I liked the jewelry box to hold her pretties, or tickets to some even they'll enjoy, he could write a poem in a card explaining their future date for those tickets. Music CDs or an i-pod stocked with music she likes doesn't have to cost a fortune but would be thoughtful.
I think you will be able to tell a lot about her based upon her reaction to this, if she's unhappy about not getting her way, it may be time to part ways or she'll bankrupt him over the years to come lol.
D.
ps, isn't it great when you enjoy your son's girlfriend. We were blessed that way, sadly my son ruined it, try not to get too close or everyone's heart breaks when things go wrong. My daughter cried over their breakup, but then she's 10 and cries if we sneeze. lol.