Read in 80s, age 9-11. Young girl in a hotel for vacation & befriends monster living in lake who is very grumpy. They develop a deep friendship. Turns out he is an alien & fellow aliens come back to see if he’s learned to love. When they take him he cries a tear that turns into a clear stone for her

There’s a girl and she’s tired of going to school and doing chores. She sees an ad in a magazine for an institute that can solve her problem by providing a copy of her that will do all the things she hates. She contacts them and they make her clone. Everyday they switch places and she goes to the institute where she can do what she wants while the clone lives her life. Then one day she decides she wants to go back to her life. She sneaks out the institute and she goes home where she sees the clone interacting with her parents. She tells the clone she’s done with her but the clone says they like me better. Then she realizes there’s a switch to turn her off on the back of her neck — and she realizes her best friend has the same switch.

I do know the book I am seeking is not the Wonderful Flight to the Mushroom Planet, but it does have a similar premise. I read it in the 50’s. It is illustrated with black and white pencil illustrations. The basic story is some children build a wooden spaceship and in the process one of them finds a flexible silvery disc that is nailed to the nose of the wooden vessel. The children discover that it can actually fly and they also discover the alien the disk belongs to, it is his own ship.
During their adventures they go to a variety of planets, one with giant jelly plants and another with bubbles, one that traps the alien who has to be rescued by the children with the very same fingernails he was so critical of earlier.