Anyone got some cool arts and crafts/DIY projects/funny stuff I can do with my cousins?

Every summer, some of my cousins come visit our family from out of state. We're like best friends so it's a lot of fun. We hang out and do fun stuff like go to the beach, go to our local amusement park, and work on arts and crafts, which might seem kinda dumb since now we're all in our teens (13-16), but it's fun. Two of us really love art, so that's how we relax after a few intense days of non-stop awesomeness. If you cant think of art projects, how about activities? We're not allowed to have water balloon fights anymore...

Every year we decorate t-shirts and sometimes we make bracelets or make stuff out of clay. In previous years, we've also made a lava lamp, used an optical illusion to make it look like our heads were expanding, and set off a fountain of Coke and Mentos geysers. Can you guys come up with some new stuff for us to do? THANK YOU!!! :D Have an awesome summer!