I have fond memories of traveling with a 6 month old in June. I did not set out to plan a trip with a 6 month old, but we had the trip planned before we knew we would have a 6 month old. We had a tradition of celebrating 5th birthdays for our kids at Disney, and child #3 was 6 months away from that special trip when child #4 (a surprise) was born.
Our older kids were 5, 9, and 10 that trip, and our baby seemed to have as much fun as everyone else. Baby slowed us down at times, but that was not necessarily a bad thing. In fact, I think that trip was harder for the newly minted 5 year old who was wore out by the heat and walking.
I agree that a good stroller and a fan are key. Our baby was accustomed to being around lots of people since she was always at events and activities for the older kids, so we did not find that the crowds and sights over stimulated her.
Our only issue was sleeping at night (she was used to grabbing naps on the go), because the baby wanted the quiet of her room and crib. I stayed with the baby in the hotel room while dad and the older ones spent nights in the parks. I am sure you will figure out the rhythm that works best for your family.