What do you enjoy doing? People find different things relaxing, so if I suggest something like read a book, and you don't enjoy reading, you would just get agitated and not relax at all.
So try to think of something you enjoy thats fairly low key (i.e. no sky diving if thats your thing as that's about the adrenalin rush). Preferibly something that makes you feel good about yourself. Do you like long baths, or watching comedy movies, or long walks in the countryside? How about re-reading a favourite novel, or trying your hand at an art or craft you've always liked the look of? Do you like having your hair cut or a beauty treatment? Maybe you enjoy meeting up with friends, or cooking a meal?
Is there something that you'd like to do for the baby coming, like knit something, or start a scrapbook or memory box, or maybe something more practical like sort out some drawers to make room for the babies' clothes? Pick something that isn't stressy, that you can either have a bit of fun and creativity with, or something that is manual like organising, that isn't difficult but keeps your hands busy.
Obviously something like meditation would be good, but it is something that takes a bit of practice and is pretty difficult to do from a standing start (e.g. Telling yourself to "Clear your mind") but there are routes into it, like guided visualisation - where you listen to a recording or person talking through a script and you imagine what they say. You can also try and imagine a blackboard or sandy beach, and any thoughts that come to mind, imagine them being written on the surface, and then either use a duster, or let a wave come and wash the thought away. Then do the same with the next thought etc. Or imagine a stream, and each thought you have is a leaf that floats past you and away.
In this way, you aren't trying to stubbornly keep your mind blank, you are just acknowledging the thoughts you are having and letting them go, because you are not your thoughts.
Like I say, meditation is useful, but you can start off by just trying to do relaxing and soothing activities that make you feel good. This should help you stay happier and more relaxed which could help your blood pressure.
Of course consider the medication that you are offered, it may help. You can also cut down/out caffeen, salt and foods that might raise your blood pressure.
Hope that helps xx