as far as i know, you can continue with the same exercises as before as long as they are not risky, i.e. horse riding, lots of weights etc. Check with your GP. I'm 25 weeks and still doing Pilates, yoga and swimming. Stretching in yoga and pilates is good for you, as long as they are gentle.. Not power yoga/pilates etc. Some pilates like lying on your tummy I can no longer do as my belly is too big, and my instructor modifies my exercises accordingly.
Walking is also great, I brisk walk from the station to work, 15 mins each way each day, though it's getting a bit longer as I'm more breathless and have started to waddle a little!!
I know I was away for 2 weeks and didnt do any exercising apart from walking and put a bit of weight. My Ob advised me to continue all my old exercises, so that I dont gain rapid weight, and he said No junk food. But I'm not a real junk food person.
But just check with your GP or Ob. Swimming and yoga is great. Oh, though you're not supposed to do butterfly or breast stroke. Crawl and backstroke is fine and is excellent as water gives you support.
If you do get hungry in between meals, try those nut and seed bars, they are great and yummy and very good for bub.
Good luck!!