Decide on a destination. It is best to keep the driving distance to a minimum. Too many things can happen while driving. You don't want a frustrating event to be remembered more than the proposal.
Arrange for transportation needs such as airline tickets, rental cars or bicycles.
Arrange for lodging. Remember that you still have a wedding to plan and possibly pay for in the end. Don't spend all your money on a fancy hotel if you are going to be financially responsible for other aspects of the wedding.
Figure out how you are going to explain the vacation. If you really want the weekend to be a surprise, involve a trusted friend of your girlfriend. It may be that the friend needs to accompany your girlfriend to the location in order to avoid suspicion.
Put some time into determining when and where you will propose. If you are unfamiliar with the area you will be visiting, ask for help from the hotel where you are staying. Generally, the staff is very knowledgeable of the area and happy to give suggestions.
You may want to propose in the middle of the weekend rather than waiting until the end. You want to have time to enjoy the newness of being engaged rather than having to rush back home so you can get to work on Monday morning.
