The easy, dumb answer is always to go for a walk. :)
I was working out before pregnancy this time, but due to illness and exhaustion am considering myself no longer regular and starting fresh. I worked out a lot with DVDs so just got the Denise Austin Fit and Firm Pregnancy one. She is way too perky, but her workouts are usually good and I already previewed it and hope to start using it today. I was going to do everything on my own, but thought it would be easier than modifying things on my own. You can always do seated arm/upper body exercises with light hand weights and/or exercise bands to keep your upper body strong (those are great when you're watching television). Depending on how far along you are and if you have any health issues/restrictions, you should not be doing anything on your back. Good luck!