I was about 6 months pregnant on one of our trips; I was considered high risk, but my doctor still wasn't worried about it. However, I didn't take it easy enough and overheated and collapsed at AK one afternoon. They actually called the paramedics, who showed up quickly. They told me to stay in a wheelchair for the rest of the trip. All but one day, I used one of those motorized scooters and it went just fine. (They were out of the motorized ones one day and DH had to push me. I was pretty embarrassed, to say the least.) We still had a wonderful trip, despite the one really scary moment.
I felt like there was a fair amount I couldn't do, but not so much as to ruin the trip. The only thing that I did that they tell you not to do while pregnant was the Kilimanjaro Safari, which I didn't think was bad at all. I did spend some time in gift shops or just sitting around people watching while DH went on rides I couldn't go on.
I'd say just go with what your doctor says. If he/she gives you the go ahead, then go and enjoy.