Think of your pregnancy as a time to focus on yourself and your body; once your baby is born, the majority of your focus will be on baby care.
If you have the time and money, treat yourself to a day at a spa. Most spas will have special packages for pregnant women. Spending time at a spa, where you can go for a gentle swim, have some pampering treatments, and enjoy being in a tranquil environment, is a great way to relax and unwind. If a spa day is out of the question, create your own at home. Run a bath, add some relaxing music (for a background soundtrack), light some candles, and relax. If you don't want to be disturbed, let your partner know and turn off the phone.
Pregnancy massage (see You are 20 Weeks and 4 Days) either from a professional or your partner, can be very therapeutic, both physically and emotionally. Another great treatment to have at this stage is a pedicure; you'll increasingly be unable to see your toes, let alone reach them, so it's a treat for someone else to take care of them for you.