Someone recommended that I leave little notes or gifts for the time I'd be gone...This past summer I left my 3 year old son for 9 days as I traveled out of the country. He was home with his dad, but was going to be with a sitter during the day or grandma would be coming to our home, depending on the day. Prior to leaving I wrote a letter for each day I would be gone, some saying that I missed them, asking them questions or telling what I would be doing that day. Also in the note was a clue telling them a certain place in the house to look for an envelope or package - a mini scavenger hunt. Some of the treasures they found included a book, music cd I borrowed from my sister, game or treat (cookies in the freezer were a hit). I hid everything before I left, and dated the letters, giving them in a pack to Dad. Each day they made a ritual of reading the letter and searching, then playing and/or eating.
When my son was younger and his dad was to be gone for several days, we made a little video of him talking to us, saying hi and he missed us and some other funny stuff. I played it each day he was gone, sometimes several times in one day! We recently came across that video on the computer and it made us laugh.
Hope that helps...oh, and our son was a bit mad at me when I returned - presumably for being gone, but within an hour or two his attitude was better. The little toy I offered immediately seemed to help.