Planet Moons:
This one is simple, just type how many moons a planet in our solar system has.
For example, How many moons orbit Earth? The correct answer is 1.

Planet Distances:
How many light years is planet x from the sun? Type your answer.

Moon Locations:
The challenger is given a grid of 9 by 3. The columns (usually letters) are the names of planets. Each planet column has 3 rows.
Four to 8 moons are placed in random positions on the top row. The user must move the moons to their respective planet. Not every column will have values, sometimes the puzzle gives multiple moons of the same planet.
For example, the challenger might have 2 moons from Jupiter, 0 from Earth, 1 from Neptune, etc.
Just because a planet has moons doesn't mean they will be on the grid. Of course each planet could have up to 3 moons, but with 4 to 8 selected that is extremely unlikely.

Planet Composition:
This one might be too close to colored buttons and too easy since it is the same outcome every time.
View the list of planets with 1 through 9 and press control plus a number to change the matter state between rocky and gaseous.
Unlike colored buttons of course it doesn't change the neighbors, and it would be more interesting to randomize the order of planets.

Wouldn't it be easier to put all the different challenges into one topic? I have noticed recently that if people have 3 ideas they become 3 topics when they should be one, game suggestions is another example.

We already have a lot of questions on Moons, planets etc.

This is unique though we can impliment. We have to check if it would be easy for developers to code. after all the complexity of the code and time for developers to write this challenges is deiding factor.

Wouldn't it be easier to put all the different challenges into one topic?

I considered for a second but decided against it. People would just make their own suggestion topics anyway, and if this giant topic did get barried then it would be hard for the developer to uncover any of the suggestions.
Using separate topics makes it easier for people including the developer to reply to specific challenges and gives a better indication of what people are interested in.
Plus a user can't write two replies in a row, they would have to edit a previous reply if they came up with new ideas. And topics don't get bumped for edits alone, so some new ideas would go unnoticed.