My daughter and her friends instituted pajama day. So for the first day of testing they all wore school appropriate pj's. She was very relaxed. Today they all decided to wear something green - so talking about fashion is putting them at rest.
While I was in the classroom I always brought healthy snacks for my class and I allowed them to chew gum during the tests as long as they didn't "snap" it or blow bubbles.
When I was in the classroom I found that giving students the choice to chew gum and bring water bottles to sip on throughout the day made them feel like the classroom was home away from home (being able to enjoy some of the same privilege as home). We had rules about both. At first, everyone wanted to chew gum and bring water bottles but it all tapered off as the year progressed.
I like the PJ theme idea. We do that the month of December when basically the whole school reads The Polar Express. The kids and adults I might add really enjoy it.