If you got both Odd Job achievements, the Arkvoodle achievement and the story progression achievement, all you have to do are the challenges. Challenges are given by Pox. He will have a red triangle over his head. Talk to him and he will give you some challenge with a time limit. Most of these are usually races, so upgrading your jetpack would make things easier.
Note that challenges aren't marked on the map.
Detailed locations can be found in one of two collectibles guides:
Complete Collection Guide