I recall reading a book in the 5th grade about a boy, a girl and a dog that end up traveling through space with an odd lizard-like alien. The kids found a piece of metal (a disk) and nailed it to their play "space ship" they have built. The alien is sort a master of disguise but he does it rather poorly. And he is out to recover the metal disc.
This weird metal gives the little club house/space ship the ability to fly and go into out into the solar system to visit some of the planets. I recall they go to Venus and almost get devoured by some type of creature. Oddly, the kids and dog have to wear clothespins on their noses to keep from perishing in space as the result of some drug the alien has given them. It is an odd plot for sure, but it was a cool book for a kid.