What they can do with her, when they pay, WHAT they are paying for, how much they pay, whether they use your tack or not, who can and cannot ride (as in one person only or they are allowed others on but need waivers)
If they are required to be there for the farrier/vet/blanketing. When they can ride, how many days, if they need to clean or replace anything, etc. If they need a helmet, if they can trailer them.
Every little thing you can think of NEEDS to be in that contract. Including their contact info.
And please have a liability clause. It's best if your lawyer writes it but you can do it yourself or pay a site to write it for you.
Also it's liability insurance that you want. If you join USEF for $35, you can have it or you can get it from an insurance company.
"Strength is the ability to use a muscle without tension"