It's best to focus on non-impact exercise, and also exercises where you are less likely to fall---remember that you're center of gravity is going to start changing pretty quickly and it will throw you off balance more than usual! I'm also in my first trimester and have hardly had the energy to exercise, but when I have I have used the elliptical machine at my gym and that was great. Just make sure you keep your heartbeat under 140 beats per minute----if you go above that, you'll be increasing your baby's heartbeat as well as will start getting into the anaerobic zone where your body will have a more difficult time supplying oxygen to your baby.
As for strengthening, it's a good time to start firming up your arms and shoulders (you'll be doing a lot of carrying 7 months from now!). I have read that some abdominal exercises can be helpful for making labor easier, but get info on this from a professional, reliable source as you also don't want to do anything that's going to put undue stress or possibly cause trauma in your abdominal region.
Good luck, and congratulations!