I played SKT but we only went to the Hill Giant's lair. It was fun, although I've heard others are better, and have actually wondered about using them as Tier2 standalone short adventures! Which of the others would you say are the best in this kind of context (ie. as standalone assault missions, ignoring the plot of SKT)? How long do you think they'd be - mostly 1-2 sessions?
My main group of 5 players doesn't move particularly fast. Plus combats with giants get a bit grindy because they have a lot of hit points. I incorporated additional story elements into each giant lair as part of a larger campaign. With all of that in mind, lairs averaged 3 to 6 sessions of about 4 hours in length. You might be able to get through them in 3 or 4 sessions -- but they're at least 2 sessions long, minimum.
Some thoughts on each:
Stone giants are my least favorite giant, so their lair was my least favorite. However, that's a matter of personal taste. The map is solid, and could make a good spot for an orc or goblin tribe -- or even a dragon.
The ice giant lair is an iceberg. Conceptually it's very cool (pun mostly not intended) and one of my favorite lairs. The iceberg's layout is simple and a careless party could turn the whole thing into a big brawl very quickly. If you want to add the complication of how to get on and off the iceberg that could be fun, too.
The fire giant lair is basically a factory. It has multiple levels as well as an automated ore bucket that together create opportunities for both stealthy infiltration and tactical combat. We had a lot of fun here. One additional factor is that there's a secondary location which is a yakfolk village that guards the entrance to the lair. I replaced the yakfolk with goblinoids, which are more to my taste. Regardless, the village makes getting into and out of the lair more complicated. You could cut it in the interest of time, if you wished.
The storm giant lair is an undersea fortress. We didn't go there in my campaign. But it's a good map and a really evocative location.
If you can find it, there's a module called the Cloud Giant's Bargain that has the player characters adventure in a floating castle. In my campaign, I used the floating castle from the last chapter of Hoard of the Dragon Queen as the epic finale to SKT. Protip: If you have an adventure in a floating castle, crash it!
Hope this is helpful.
(EDITED for clarity.)