Build a cardboard contraption

What can I use instead of wooden skewers, drinking straws, or bottle caps if I can't find them?

If you don't have the wooden skewers from 'cut the two wooden skewers', use sharpened pencils or thin dowels as axles, replace the 'cut two drinking straw pieces' bearings with rolled paper tubes or short pen caps, and swap bottle-cap wheels with LEGO/toy wheels or coins taped on.

My wheels won't spin freely or the car goes crooked — how do I fix it?

If wheels bind or the car tracks crooked, re-tape and realign the straw bearings under the chassis 'about 3 cm in from each short end' so they line up across the width, trim or sand skewer ends to reduce friction through the straws, and reposition the small piece of tape on each skewer end so the wheels don't rub or slide off.

How can I adapt this activity for younger or older kids?

For younger kids have an adult pre-measure and cut the 20 cm by 10 cm chassis, pre-cut straw bearings and skewer pieces, and let them slide on wheels and loop the rubber band, while older kids can measure and cut the chassis themselves, experiment with more rubber-band wraps and different toothpick-hook designs, or strengthen the axle attachment.

How can we improve or personalize the rubber-band car to go farther or look cooler?

To extend and personalize the car try moving the straw bearings slightly from the 'about 3 cm' marks to change the wheelbase for stability, use a longer or additional rubber band when you 'attach a rubber band' for more power, replace the taped toothpick with a sturdier hook, and decorate the cardboard chassis before running.