You can get wheelchair umbrellas like these that clamp onto a wheelchair.
http://www.amazon.com/Rolyn-Prest-Wheelchair-Umbrella/dp/B007G4VNSQ
http://www.amazon.com/Paraplis-Umbrella-Holder-Wheelchair-E10001E10001/dp/B002LDFTO6
There is a generic stroller shade I have seen that might work on a wheelchair:
http://www.amazon.com/RayShade®-Pro...1367174768&sr=1-1&keywords=Stroller+ray+shade
We tried an umbrella one once on DD's wheelchair, but it was always in the way of either her or the person pushing her. We've had much better luck just having someone walk beside her to shade her.
ALSO, since overheating is a concern for you, it's important to keep in mind that many of the sunshade products trap heat and don't allow air circulation. Other suggestions:
- many people recommend things from a company called Frog Toggs. They have items called Chilly Toggs that are meant to cool people.
http://www.froggtoggs.com/
- you can also buy cooling vests that actively cool people. They are made by different companies and work in various ways. They are generally quite expensive, but for some people they are absolutely necessary. You MAY be able to get insurance coverage or be able to use a medical spending account to purchase them with a doctor's prescription and justification of need.
http://www.stacoolvest.com/
http://www.glaciertek.com/
http://www.arcticheatusa.com/
- go as early as possible and stay a few hours, until about 11 or so when it is starting to get warmer and the sun is higher and more or an issue.
- go back to your hotel, site see at other resorts or plan a sit down lunch if you are planning to be at a park later when it is hot ( sun is most intense between 10 and 2)
- come back to the park in early evening when it has started to cool off.
- plan your touring to do outdoor things if possible during the cooler times and save the longer, indoor air conditioned things for later when it is warmer.
- using a touring plan can help a lot with shortening waits and also with crowds - the less people in the park/area of the park you are in, the easier it is to move around.