We left our 18 month old with my parents when we went to Spain for 2 weeks. We had lived with them for nearly a year, while we relocated back to the area, so he was very comfortable with them.
I was fine the first week, but by the end of it, I was a mess - I missed him and wanted to go home. We talked to him nearly every day on the phone and tried to call before bedtime.
He was almost potty trained, but refused to go on the toliet after we returned. It was somewhat of a punishment for us - since he would still go on the toliet for my parents, but not us.
We always leave a gaurdianship letter, signed by both my husband and I - along with the insurance card. I figure the more we have covered, the less will probably go wrong!
Keeping him busy with normal routine stuff is probably the most important. Just how he likes certain food prepared, when does he get his hair washed in the bath - at the beginning or at the end? Just keeping life normal will help.
Sounds like a good weekend for all involved! Good luck and have fun!