You stop him before he gets to the entrance.  “Hey, you don’t want to be road pizza, do you?  Let’s be careful.”  You sneak into the cave and since you’re going slowly, you avoid all manner of pitfalls that you could have ran right into.  When you reach the back of the cave, you find a bag of gold coins… near another entrance that points directly towards your friend’s fort.  That’s how you saw that glimmer.

You pick up the coins and sneak back out unscathed.  You follow the prints back to your bikes, ride home, and show them to your parents (not telling them that you found them in a cave, in the woods, narrowly avoiding many types of peril).  They get them appraised and sure enough, they’re very old and valuable coins that a collector pays dearly for.

Most of the money goes into your college funds (booooring), but your parents give you a bit of it to spend as you want, and you buy the power glove and your friend buys Bad Street Brawler and pretty much the entire candy store, and you FINALLY have that video game marathon you never got around to before.

Of course, you keep two of the coins for yourselves so you’ll never forget such a choice day for the rest of your lives.

THE END of a very successful adventure.  Try again, if you’d like.