In order to relax while you are away you have to leave your baby behind with someone you trust. Even if you have a regular babysitter or daycare provider, it may be a better idea to choose a friend or family member for an overnight experience. Before the overnight, try a few visits with this person just staying for a few hours. Make sure this person has experience caring for an infant and make sure they are familiar with the nuances of your child. What might be an emergency for one child could be perfectly normal for another. If your overnight caregiver is familiar with your child's eating and sleeping schedule, and make sure they know how to handle any unusual medical issues.
It will be easier to relax and enjoy yourself during your trip away if you check in a few times. You might feel like you have to completely separate during your weekend away and focus only on one another. This is not going to make either parent happy, so allow a chance to call home and check in periodically. There is no reason to do it every hour, but a few calls will help you feel better about being away. Set up an agreement with your caregiver ahead of time for them to call if there is an emergency.
Once you have set up care arrangements for your child, it will be time to decide where to go. Your goal is to get far enough away from home that you can let go and relax, but still be within driving distance should something happen. Most towns have a few bed and breakfasts within a few hours, providing the perfect getaway for new parents.
After you have arrived, do your best to recapture some of the magic in your marriage that was there before you were parents. Enjoy a quiet dinner together, talk about topics other than baby and focus on what it was that brought you together as a couple in the first place.