You have to land on mecury then get in low and high mecury orbit then the same for Venus earth and our moons Mars and it's moons then around Jupiter and all it's moons then back to earth. Rule: no mod.
Add Pluto, have the player build a gateway with on docking port and one crew module with a capsule. Then have them land on Pluto, get back to the gateway, and get the gateway back to earth. Only rule: no infinite build area or cheats.
Those challenges you post are very ambitious! However you have to know that we set as a rule that each member that proposes a challenge has to do it himself first. Most people won't take it in serious otherwise.