When best friends Dak Smyth and Sera Froste discover the key to time travel — the Infinity Ring — they're swept up in a centuries-long secret war for the fate of humankind. History has gone disastrously off course — and it's up to them to fix it!
Infinity Ring Book Five: Cave of Wonders by Matthew J. Kirby
After their grandmother dies, Amy and Dan Cahill discover that they are part of the most powerful family in history. The source of this power is hidden in 39 Clues scattered around the world. Amy, Dan and other Cahills are given a choice: Take a million dollars and walk away, or compete to find the ultimate prize.