With the release of Storm King's Thunder, we got a new adventure but also a new resource to shamelessly steal from for our homebrew. I did it before with Curse of Strahd but I think that going forward I'll continue to do this with every new adventure. With that in mind, here are 3 things that can be easily and effectively repurposed.
The free first chapter of the adventure, found here, provides an introduction to the storyline. It also gives a series of encounters inside a city. As it is, it can be run as a goblin attack on a city followed by an orc attack (completely removing giants from the equation). The one part that's left hard to explain in this case is the rocks (you can change the description of character deaths from being crushed by rocks to being shot with arrows). You can also easily change the goblins to guards of an opposing army.
The encounter in general is quite nice in my opinion. Part of the reason is that it occurs on a ship. As it is you can very easily replace King Hekaton with another similar creature (dragon comes to mind) and still keep the encounter largely intact. The betrayal of the crew is a nice touch and easily kept when reusing. You can also choose to replace King Hekaton with a special chest containing a magic item needed for your quest. This chest should have magic properties similar to the chains that bind King Hekaton.
The Savage Frontier
The adventure contains a nice long description of the area, what creatures and characters live within it, as well as some interesting plot hooks. The Wight Brothers are a personal favourite. It's a short little description and location but there is enough there to give me inspiration for a nice wight themed adventure. The area itself, up in the cold north, can be easily used as the backdrop for your own themed adventure. Want to do an evil undead army attacking? It still works. Want to do a Tyranny of Dragons style storyline based in the north? Between the location and the dragons, you got it. It's a bit harder to point to one thing that can be reused in this case since we are talking about a large area and quite a few different characters. However, that vastness and potential is what makes it valuable when plotting a campaign. The adventure also adds some details to the Sword Coast Adventurer's Guide, adding to the Forgotten Realms and can be used to add to previously published adventures.
Storm King's Thunder is heavily based around giants. Removing giants from encounters and sections of the adventure becomes quite difficult as a result. However, it will be much easier to adapt and require far less work if giants feature prominently in your campaign.