Everything I was doing before I was pregnant. So, 10-15km cycle each day, and a long one on the weekends, 2 x 1km swims per week, and as many walks to the supermarket as the pantry required.
At about 10 weeks I started to lose speed and strength on the bike, and at about 20 I cut back to 1 x 1.5km swim and dropped some of the longer bike rides, but picked up a physio-led exercise class At 34 weeks I hung up the helmet, and now at 36 weeks, I still swim 1km once a week, and waddle as far as I need to - usually just the bus stop or the local shop, and only do one thing per day.

If you are feeling dizzy, you need to STOP. I had the all-clear from my Doc to do everything I did before, with the advice that my body would tell me if it was too much. I knew not to push myself, but getting "the pinks" or dizziness was a good signal to settle down. It only happened once.