Organization
If you have elderly or disabled family members that will be attending, you might want to consider creating a special subcommittee just for this purpose.  This subcommittee would be responsible for determining if there are any family members with special needs (disabilities, special diets, etc).  This is done easily enough by adding the following comment to the pre-reunion survey, reunion announcement, or other early mailing... "If any family member has special needs that the reunion committee should be aware of, please contact John Doe".  I assume Mister Spiffy doesn't need to tell you to replace "John Doe" with the name of the subcommittee chairman, and to add a way to contact him or her.

The subcommittee chairman should make a list of these special needs, and present them to the main reunion committee who can decide how to handle them.   The subcommittee is then responsible for following through on whatever decision is made.

Reunion Types and Locations
One of the big considerations is the type (and location) of your reunion, and whether it is feasible for elderly or disabled family members.  Now Mister Spiffy isn't saying not to have good active reunions, just make sure that there is something to do for everyone.  For example, if you want to have a reunion at a water park, make sure that those who can't (or don't want to) slide have something to do besides sitting around getting sunburned.  Make sure there is plenty of shade, and comfortable places to sit around and chat.  Most importantly, if you are having a reunion at a public or private site, make sure the facilities are accessible to all family members who will be attending.

Games and Activities
Make sure that there are at least a few activities that anyone can participate in, including elderly or disabled family members.  Have a deck of cards or some board games for the less mobile family members.  For more ideas, you might want to check out the book Popular Activities & Games for Blind, Visually Impaired & Disabled People.

Food
One item that many people overlook when planning a reunion is the special diets of some family members.  It isn't necessary to plan the entire menu around a person's special diet, just make sure nobody has to go hungry (the number one family reunion sin, according to Mister Spiffy).  Here is the ultimate resource for people on special diets.

Travel
One of the bigger considerations for elderly or disabled family members is travel to and from the reunion (as well as any necessary travel while at the reunion).   Although travel to the reunion is usually the responsibility of the immediate family itself, here are some tips on traveling with various disabilities, which you may want to pass on to the interested party.   In addition, here are some other travel resources for the elderly or disabled.

Other Resources
Yellow Pages for the disabled and elderly
Phone numbers for associations and organizations for the disabled
Addresses for associations and organizations for the disabled