I think a surprise trip is a fantastic idea! I have always wanted my fiance/husband to book a trip all on his own, contact my place of employment and just surprise me with the tickets!!!! Note though -- my dad surprised my mom, and while she was VERY happy she felt she didn't have a lot of time to prepare/get excited (he told her the day before), so give a bit of time before so she can research places to go and have an input in the itinerary!!
If it doesn't happen, jewelery is also fantastic, and customary! Let us know what you decide!