Bring along some brightly colored ribbon, or a bandana to tie on one of the handlebars - in a sea of identical rental wheelchairs, yours will stand out quicker.
Also a Godsend if your chair has to be (will be) moved by a CM while you are in a show/attraction. So, don't freak out if you come outside and the chair has been moved; this is a normal thing for the CMs to move strollers,
ECVs and wheelchairs around to keep walkways as open as possible, and to fill in the empty spots created as others leave.
You can easily put a backpack on the back of the wheelchair; just don't put anything in there that you don't mind losing if you aren't going to take it with you - or bring along some zipties from home to keep it attached.
You may want to consider bringing or renting a gel cushion for your Mom-in-law; it can actually become painful to sit too long in the "sling" style seat of a wheelchair.
Bring along a towel from the room to drape over the seat and armrests while you are inside; black plastic and the hot Florida sun are a bad combo.
Bike gloves/golf gloves - the reason I have always recommended bringing a set of these is because the older style hand grips (think of the shiny black plastic style hand grips that you will push the chair with from behind) *will* cause blisters if your hands are prone to sweating in the heat. You don't need a lot of protection; just something to help prevent the "pushers" from winding up with sore, blistered hands.
Battery operated lights are great, as are glow sticks - just remember if you are going to fold up the chair, string lights could get caught in the folding mechanism.
Remember that you will load on to buses first, but get off last, so don't be surprised! She can sit in the wheelchair if she wants to; the bus driver should buckle her in with a special seat belt that goes around the wheelchair, as well as the standard tie-downs. If she wants to walk up and down the bus ramp, that's fine - but again, you will do that first, and then must wait for the driver to "kneel" the bus and deploy the ramp after everyone has gotten off.
Remember that she won't be able to hear much of the conversation if everyone in your party is behind her - so try to include her as much as you can. It can get lonely; often people don't realize how isolated you can feel sitting down when everyone else in your group is standing.
Have someone try to walk beside her, as well as in front to create a "buffer" zone; this will hopefully minimize the number of people who bump into her, or walk directly in front the of the chair, causing problems for the pusher. Remember that little kids have zero idea of how physics work - they don't realize that you can't stop on a dime - so they tend to dart in front of wheelchairs and ECVs and strollers like tiny little crazy people.
