Anything that won't put pressure on your stomach.
There are people who lift weights, run marathons, cross country, anything during pregnancy.
Its what your body can handle.
What a lot of women do is they wait until they get.pregnant to decide to get fit and healthy. But because they didn't workout before they got pregnant, they're limited on what they can do.
If you're like that, walk on the treadmill. Cycle, do steps, squats are good.
Weighted squats are great. Lunges, crunches, planks, all great!!
Listen to your body. You'll know if you're pushing yourself too hard.
Here's some advice on exercising safely during pregnancy from MedHelp.org and the I'm Expecting app.
http://www.medhelp.org/pregnancy-health/articles/How-to-Exercise-Safely-When-Youre-Pregnant/2219
You can also find it in the Week by Week section of the I'm Expecting app -- check Week 12.
You can also talk to your ob to see what they say. But like roflbecca said. A lot of women wait until they get pregnant to decide to workout. So just the basics are what you can do. Run, squats, lunges, planks. But women who worked out regularly before getting pregnant might be able to do more with weights. But I would definitely recommend talking to your doctor.
I do a lot of yoga and walking