5th Grade: Viridian Room
I am doing a flash game in class today. It is called Viridian room. I did another game, "Crimson Room" in class last year and my students still ask me today to do it again.
Crimson Room was much easier, but I think the students like the challenge of Viridian Room. Also only a few students have played the game while almost every student has already solved Crimson Room. Here's the link to the site.
If you need a walkthrough look here. I suggest you let 2 kids up at the computer at a time and then get the students to tell those two students where to click and go in the room. It helps to have the walkthrough so you can also help direct the kids and keep the game moving.