My wife was going through her chemo treatment when we had our family reunion in 2000. She did just fine. We made sure to take our time and not try to do too much in any one day and we also made sure to allow plenty of afternoon time for rest and naps. Sunscreen and protective clothing (especially a shady hat) were essential too.
We very seriously considered not going, but she made the decision to go, mostly because it was a long-planned family reunion. When it was all over, she (and all of us) were glad she went because the trip really brightened up her life.
BTW - her schedule for chemo worked out perfectly, she didn't need to have a treatment while there. We worried that we might have to have a treatment there and had started to look into how to do that. Our docs here mentioned some clinics there that we might have gone to, but, as I said, the schedule worked out that we didn't need to do that.
As an aside. On our last night there, we wanted to take one last trip to the Magic Kingdom but everyone was too worn out, so she and I went over to ride the mountains one last time and watch the fireworks. It's a great memory!