I do not claim to be an expert on this topic, but I have done it a few times. On this trip, it is just me and my dad, so I can focus more on his needs. He is almost 70, so not too terribly old, but he does have a set of challenges from his Parkinson's Disease. I also spent a few years taking care of my first husband, who became quite disabled due to his brain cancer. So... here are a few things that have made life a little easier on vacation.
Mind the sleep schedule. A healthy young person can get away with less sleep, but don't push it for them. No red eye flights. Don't plan things for very early or very late. I'm feeling better when I get enough sleep, and it is compounded for an older or disabled person. For this flight out, I had a hard decision between leaving early in the morning or arriving late at night. Coming from the west, I chose to arrive later because 11 pm in Virginia was really only 9pm for us. That one backfired on me due to long delays in Chicago. Grr. We didn't get to the hotel until about 2 am! Which leads directly to the next point.
Travel easy. This is not the time to cut all corners to get the best price. Do that when you are taking you after-college backpacking trip through Europe. Take direct flights with no layovers. Get a hotel close to your destination. Don't sweat all the small stuff. Minimize walking. Oh, and get a hotel room with a kitchenette!
Don't forget the medicines. Most meds need to be taken on a fairly strict schedule. My dad has one he has to take four times each day. During our five-gate switcheroo at the Chicago airport, I noticed Dad starting to get shaky, and remembered that we had missed a dose of medication! Being away from home is stressful enough. Don't mess up the system further by forgetting the drugs. Know which medications need to be taken with food, or an on empty stomach, and be prepared with snacks. And be aware that pain will likely be worse, due to stress and unusual activities. Take pain pills along.
Keep it low stress and cheerful. Did I mention our delays in Chicago? It was a mess. An entire runway was closed, and it threw off the whole airport. Everyone was shuffled around, and everyone was frustrated. But I have learned (the hard way!) that if I lose my cool, I have not only my own emotions to deal with, but also a breakdown from my charge. So we did a lot of people watching. We played a game out of trying to get others to smile. We made jokes with fellow travelers. By keeping my frustrations in check, I helped my dad to not get stressed out, and the whole situation went ever so much better
Ask for accommodations. Most places have accommodations for the elderly or those with disabilities, but you have to ask for them. Ask to be preboarded on the plane. Ask for an handicapped-accessible hotel room. Ask to be seated near the aisle in the auditorium. Ask for an elevator, or for the closest table. Most people are more than happy to help -and they actually are required to make accommodations! I am ever so grateful to the cheerful airport worker who ran down the concourse, pushing my dad in a wheelchair, so we could make a connecting flight.
Slow down and allow for breaks. I spent most of one vacation in a hotel room, missing out on day trips with the rest of my group, so my ailing husband could sleep. Traveling is stressful, and you need more rest. I'm typing this right now as my dad takes a nap. Allow for time, especially in the middle of the day, or after a long walk, to rest or nap. Plan a resting day after a busy day. And walk slower so your companion doesn't try to kill themselves keeping up with you.
Eat well. Your body needs better nourishment when it is stressed. Avoid the rich and fatty foods if you can, or at least eat them sparingly. Pack lunches for on-the-go times. Make healthy choices from the restaurant menu. Get good snacks of fruit from the grocery store. Prepare your own food if you can. We got a hotel room with a kitchenette and it has been wonderful. We even had hot cookies baked in a frying pan! If you have healthy food available - and his requires advance planning - you will be less tempted to go through the drive thru or get something unhealthy from the gas station. Make sure to eat enough fiber. Nothing will ruin a vacation faster than getting sick, or feeling constipated and miserable.
Stay familiar. Bring comfort things from home. Keep the same medication and sleep schedule as much as you can. Eat the same kinds of things as you can. Don't leave your companion alone in a strange place. Do things they usually like to do. We're watching tv in the evenings as a way to unwind on this vacation even though I don't usually... but Dad does.
Take care of yourself. Make sure you get enough sleep and the things you need. You know how the airline attendants always tell you to put on your oxygen mask first. Do. You won't be able to help much if you are not well.
Have fun. You are on vacation to have a good time, so have a good time!