Things your dog's owner should go through with you before handing over the terrierist for a summer holiday. :)
Updates on when dog was last wormed/flea treated and date that next treatment is due. If this falls in your period of care, information and instructions on what products, and how funded.
MOST IMPORTANT - vet details if you are going to the same vet, or if you are too far away, agree on what surgery you will be temporarily registered with. Dog insurance - policy details and info on how to claim in the case of emergency. Agree a protocol for vet treatment - who is going to be responsible for paying, and making the decision about when/whether to take.
Dog ID - is he chipped? If so, update chip details for the summer. Consider having a new ID tag made with your mobile number on, not his old number. Pre-register him on DogsLost website, so that if he decides to leg it, his details can be distributed nationwide within an hour. Also put the vet's number and the local dog warden number into your mobile phone.
Discuss with his owners feeding - times, what he eats, any allergies, what treats he is allowed. Transport - how does he travel in the car? Harness, cage, in boot etc. Toys - favourite squeaky ball, Kong, etc.
Grooming - how often? What with? Will he need a visit to a professional while he is with you?
I'd have a good check round your garden - make sure fences are secure and remind family members about the importance of keeping doors and gates securely closed.