This is, of course, all my opinion and based on my relatively limited experience, so take with as many grains of salt as you please. :)
I'm also going to continue referencing my block tower metaphor, so those who missed yesterday's post might want to read at least the first bit so they have some idea what I'm on about.
So, making middles fun (not just for the writer, but the reader too)...
1. Build upward.
This may seem like a pretty obvious one, but I've read an awful lot of books where things happen that don't actually progress any of the story threads, they just provide a little momentary conflict that has little to do with the rest of the story while the author tries to figure out how to get to the next important part. Or scenes occur that rehash conflicts the characters have dealt with previously. That's not building towers; that's chucking random blocks at the reader, or taking a block you've already set on the tower off and putting it on again. Neither of which gives the impression you know what in heck you're trying to create.
Middles sag when readers feel that they're not getting anywhere--and start wondering if the book is ever going to get somewhere they want to be. So make sure each scene you show moves one or more of your plot threads closer to its conclusion. That way you'll keep the story's forward momentum, and the sense that things are developing and if we keep reading we'll find out what and how.
You'll also find that by building upward, you'll automatically increase the tension with each development. (Consider how, with actual block towers, the tower gets ever more fragile with each block added to the top--and anyone watching anticipates it toppling even more.) As each plot thread gets further from where it started, each step should naturally get more difficult, the stakes higher. Because as things change, your characters are getting further and further from the things they were used to, from that solid ground they had at the beginning--which means they have further and further to fall if things go wrong.
(I am assuming, of course, that your plot threads involve conflict and obstacles that prevent them from leaping straight from their starting point to their end. For more thoughts adding tension to stories, I have a big long post about that.)
2. Let change be good.
If a reader can look at the beginnings of the threads you've laid out, and immediately see the exact path they'll take to the end... well, that's not very exciting, is it? Certainly doesn't give much incentive to read on.
We've already established that with each scene, the threads of your plot should be changing as they move forward. Have at least some of those changes be unpredictable. Life very rarely follows a completely straight path, so why would your characters' lives do so? A few twists and turns can add to the tension and keep readers on their toes, and keep that middle section exciting while still moving toward the climax. Whenever it seems "natural" to add a certain block to the tower, stop and ask yourself--is it because that really is the best direction for this thread to take? Or because you've seen so many stories built the same way before? If it's the latter, see if you can come up with something different that fits your situation and characters, and isn't so familiar.
(I find it easiest to work out those twists and turns before I even start writing, while I'm figuring out my plot threads and outlining, but that's just me.)
3. Remember that characters change, too.
Unless you're writing a very plot-centric story, which is more about the characters just getting through the story than having them grow (or a very depressing story, which is about how the characters fail to grow), your main character probably has an arc. Some change they have to undergo before they reach the climax, in order for them to succeed. But to be believable and interesting, that change isn't going to happen in one scene, just as your plot threads aren't going to jump from beginning to end in one step. It should be happening in increments throughout the story, in tandem with the developments of the plot.
How can this help your middle? Well, in every arc there is a mid-point.* A point where the character makes the shift from being more the way they started to being more the way they will be at the end. The point, say, where the character goes from being 51% mean (or lazy/scared/insecure/whatever) and 49% nice (or motivated/brave/confident/whatever) to 51% nice and 49% mean.**
A character who's more mean than nice (or whatever) is going to make different decisions and have different goals--or at least come at their goals from different angles--than a character who's more nice than mean. Which means this is your opportunity to shift those towers you've been building in a new and exciting direction (see point #2). And if you've been pacing your character's arc at a fairly even rate, that switch is probably going to happen right in the middle of your middle!
I find this makes for a great way to break up that long stretch between beginning and end--by seeing it as two parts instead of one, the middle pre-character-shift and the middle post-character-shift. The first half of your middle is not just developing all those plot threads, but building toward pushing your main character over that mid-point, and the second half lets you show how that change is affecting them (as they continue to change, and become more and more the new them) while they have to struggle both with their new way of looking at things (which they may not always be happy about) and with all the problems they already had (their reactions and attempted solutions to which will undoubtedly start to change as well).
So, that's how I try to keep my middles interesting. If you have any tips of your own to share, comment away!
*When people talk about arc structure you will sometimes find references to a "point of no return" in the middle of the second act; this is very similar to what I'm talking about here, though I'm talking about it from a very character-development-centric angle.
**I do realize this is a very simplified version of character development, but bear with me for the sake of making a example that doesn't take five pages to explain.