One idea could be that they travel back to the time when their parents first met. They could accidentally disrupt the meeting and then have to work hard to make sure their parents still fall in love. For example, they might cause a small accident that makes their mother late for the meeting, but then they find a creative way to get her there on time.
They might time travel to a pre - dragon era. There, they face challenges like having to hide their knowledge of dragons as dragons are considered mythical beasts in that time. They also have to figure out how to get back to their own time without changing the course of history too much.