I just remembered some of my OLD (like 2nd grade old) series ideas that I had.
When two twins/siblings have the same dream on the same day about a historical event, they have to travel back in time to change the events to what really happens or else the future will change. Their enemy is Mr.Tickity (oh god why) and he changes things in the past (like making England win the Revolutionary War instead of America).
Someone moves into a new house and discover that the key their grandmother gave them, opens up a room with a door that hold great secrets. To open the door, she must find 100 stone slates that fit. A mysterious person, doesn't want them to find it and tries to hide/steal the stones.
One cold winter, a person discovers a room that brings her to summer, spring, and fall! She can constantly go back and forth, but nobody can know about it.
This is all I have right now but i'll keep updating when I remember more!