Need help with adventures in Stygia, Muspelheim and Jotunheim

Hi guys, I never run a campaign for higher level players ... until now. I wanted something fantastic, to show them other planes, where they have never been. I decided to write a little story (5-6 sessions) that starts on Material plane, brings them to Muspelheim, the land of fire giants, continues to Jotunheim and ends in Stygia.

Then, I realized that I don't really know how to make those places unique and eye catching. What encounters and mechanics should I implement? Do you have any materials that I can use to get inspiration?