The promise: After you book a hotel, this site tracks the latest rates and rebooks you if the price drops.
Available on: Website
Unlike sites that just alert you when a rate changes, Tingo automatically locks in the savings.
The site offers this service only on refundable rooms, though, so your credit card is charged when you reserve. But if you cancel or a lower rate is found, the difference is returned to your card.
If there's an extra $10,000 in your budget, consider investing in low-cost index funds, buying a reliable used car or taking a European river cruise.