What's the minimum you need to unlock before you can go off and have more open-ended fun?

I put over a 100 hours into this game when it came out, but it's been a while. My brother is starting to play it but is less interested in the linear story.

What do you recommend unlocking (or what do you need to unlock) before you can start doing more of the open-world/open-ended stuff? So far I'm remembering he needs to unlock:

• All the lives
• Home base
• Ginormiosa
• Strangelings and the ability to have buddies
• The sort of cavern thing connected to your base (I can't remember the name) that's part of the main story
• Treasure Groves

What else am I missing? Or are there any good existing guides for this?