Progress

Nora thinks that a spatula would make a good roller coaster car.  Silas is going to cut open an aluminum can with a razor to make a seat to ride the roller coaster.  Avery is leaning toward the spatula idea, but he might try a plastic cereal bowl if he can find one.  They've collected enough cans now to complete the roller coaster, but they still don't know what to use to keep the cars on the track.  PVC pipe?  That's all they've been able to come up with so far, though they haven't been able to locate any.  Any ideas?

Aluminum

Nora set down her snickerdoodle.  "How many cans of spinach water have you had, Avery?" 
"That's my fourth one today," said Avery.  "I know it's a lot, but I'm saving up the cans to build a roller coaster in the next tree over." 
"Cool!" said Silas.  "Do you have any mint water?"
"Here you go."  Avery tossed Silas a can of mint water and stacked his now-empty spinach water can with the other empty ones.  He had been saving them up for several weeks now and had collected over 50 aluminum cans. 
"Did you drink all of THOSE, too?" asked Nora, pointing at the collection.
"No, I snagged some of them from the recycling bins at the park."
"Are we ready to start the roller coaster yet?" asked Silas as he tossed his now-empty can toward the stack.
"Wait, Silas, don't!!" Avery started.  But it was too late.  Aluminum cans rolled in all directions, lodging in the crooks of the branches below them as well as dropping to the ground.
"Great," said Nora.  "Now people will think this is an aluminum-can tree."
Avery blinked at her.  "You're picking those up," he told Silas, "and then I guess we'll start it now."
"All of them??" Silas groaned.
"At least get the ones on the ground.  I'll get what I can carry from here and meet you in the other tree."
"Hooray!" Silas leaped down the tree to pick up cans while Nora and Avery gathered the cans from the tree and carried them next door.  Together, they fastened the cans between the knobby branches to build the hills and loops of a roller coaster right in the tree. 
"That looks good," said Avery, "but I want to make it longer.  Let's collect some more cans and add to it later."
"Yeah, then it will be the BEST!" said Silas.
"How will we ride it?"  asked Nora.  "What will we use for the track and the cars?"
"Well, I don't know yet," said Avery.  "Do you have any ideas?"
"I'll think about it."
"Ok, we'll all think about those things while we collect more cans.  Agreed?" 
"Yes!" shouted Silas and Nora.
"Great.  Now let's get home for dinner.  I smell dandelion seeds..."
Nora bounded straight across to the home tree while Avery and Silas watched in amazement.
"Yep," said Silas, "definitely her favorite."
"I would say so," chimed in Avery.  He and Silas laughed as they slid down the trunk and then climbed up to their home.  And dinner was sweet.