Hi S.,
I'm very happy for you that you and your husband will be taking time away together. We are better spouses and parents when we take time to nurture our relationship; make a habit of doing it. Not only for your marriage but adjusting to little bits of time away from you is very good for your child. It will help him to be a well-adjusted child. Having been a teacher of this age group, I can tell you this is a character trait you always want to develop.
Now to your question: Children this age have a very poor concept of time. (It doesn't help when we say "just a minute" to them and that minute can sometimes mean a moment and sometimes mean 1/2 hour.) :) Make a 5 day over-sized calendar for him. 5 big squares. Between each square, draw a picture of a bed (or better yet, a picture of him sleeping). When you get to grandma's, hang it up and go through it with him day by day. "This square is today. Today we packed your suitcase and we drove to grandma's house. Today Mommy and Daddy are going to go away and you get to stay here with Grandma. When its time for bed tonight, Grandma will give you a crayon for you to cross off today because it will be all done. Then you go to sleep. When you wake up, it will be tomorrow. (point to the next square) Tomorrow you get to..." If there are special activities planned for a certain day, a picture of that activity can be drawn in that square. Having a visual reminder of the days is completely understandable to a child this age.
Something else my husband and I have always done when we're gone (and continue to do even though our children are older), is we make up a bag for each day we're gone. We use the white lunch bags you can buy just about anywhere. We write the day on each bag. Each morning, our children get to open a bag. Sometimes its some candy for each and sometimes its a new video to share that night. It gives them something to look forward to each day AND its been the alternative to bringing home some "junky souvenior" from wherever we've been.
And, while I'm on a roll here: this doesn't pertain to you right now but if others read this... whenever someone from our family will be gone (the kids to church camp, my husband on biz, etc), those remaining at home will make up an envelope for each day they will be gone. When they are about to leave, we hand them their stack of dated envelopes so they have something to open each day while they are gone. Each envelope will be a note, a drawing, etc from various family members. Often it simply says, "we love you and we're praying for you".
I hope you have a fabulous time away together. Take off your parent "hat" and become totally focused on each other. Hey, and put some thought into that time together as well. Prepare to surprise him. It doesn't need to be fancy. There's an old song with this line: "she was standing in the kitchen with nothing but an apron on". One time I packed an old Menards apron, suprised my husband, that's for sure!!! :)
K.