Students are always excited to tell their friends, and potential new friends, what happened on their summer break. So any art project that allows them to show their own experiences can be beneficial. Have students make a summer break memory project. Tell them to make a piece of art that will always remind them of their best summer break memory.
If you are willing to let your students have the run of the art supplies, you can prepare yourself by learning what will be the most popular medium for students during the year.
Getting to know the students, and allowing them to know each other, can be a good basis for a project for the first day. A classic project is to assign a self-portrait that does not in any way involve a picture of a person--tell students to represent themselves with other images. Then they will spend the class explaining, or being artistically mysterious about, why they chose particular images. This can be a collage or a drawing project, or even a graphic arts project if the equipment is available.
Putting students back in a scholarly frame of mind can be difficult, but giving them a goal is a good solution to this problem. Tell them that they are going to make a simple project of some kind, depending on the age and genre of the class. The project can be a still life if it is a drawing class, a sculpture for a sculpting class, and so on. Then collect the projects and save them. Tell the students that they will repeat the project on the last day of school--not for a grade--but just to allow them to see their own improvement.