I have been running the same campaign with some of the same players for over twenty years and it is hard to keep that sense of wonder everyone had when you all first started playing.
Thankfully the game we play has evolved and this has brought with it new challenges and different, more interesting ways to build and play characters and a new setting to explore.
Golarion is both familiar and yet different enough that it has got me interested in playing or running the various adventure paths and Pathfinder Society adventures set there, but I still run my homebrewed setting that steals unashamedly from my favourite books, films, comics and games.
To maintain variety in my own game I vary the locale for the adventures, vary the premise or introduce new twists like critical hits and fumbles, hero points and story cards.
I try not to run too much of one kind of adventure even though it is often difficult to move outside my comfort zone. I also take guidance from my players so that they have a hand in shaping the direction of the game.
If you can all agree on what you want, or at least reach a fair compromise, then everyone should have a game they can enjoy.
I also find that getting out and gaming with different people helps add new ideas to my own gaming bag of tricks.
Huh, couldn't dissagree more with the a goblin cave is a goblin cave bit...
As for keeping things different, I go for a different area of the same world, rather than a completely different world. Since Pathfinder came out my world of choice is Golarion, so to mix things up, after going through Rise of the Runelord, I'm going to do a mix up of Age of Worms and the Falcon's Hollow series. Going from a home base where you actually care about the NPCs/locale, to a home base where everyone is a worthless vile wretch, bearly worthy of the cross bow bolt you want to put between their eyes...makes for some sense of change.
Sense of exploration can mean many things. Giving the (semi)intelligent monsters the party fight their own well-thought-out culture can do wonders in this regard..."So we figured out the baby-kicker clan goblins sneak into human seatlements and steal babies...but why go through all the hassle and risk?"
Beyond that, give them a history to get wrapped up in...Don't just let them find a +3 defender long sword in the dragon's hord...let them find Griswald, the town's blacksmith's, father's magic long sword...and either have him offer to buy the family heirloom off of them (or trade for other items) and ask them to track down the rest of his father's long lost adventuring kit.
Custom world, custom culturesCustom made monsters
edited existing monsters
steal ruthlessly from any book, movie, or TV show that might be appropriate and fun
All of these are in my arsenal.
I am blessed in that I am attended by many Muses, and do not find myself short on ideas. Almost anything can be an inspiration.
I love building things, too, so odd gizmos of my own design are common, as are monsters built from scratch.
But to me, the most important thing is keeping an open mind. It's a cliche to say we ought to remain flexible, but it bears repeating.
Things like crashed spaceships, gunpowder, radioactivity, psycho babble, and other modern themes can sometimes become controversial topics for gamers, but if you stay on your toes, they can be very effectively adapted and pose a good challenge to every aspect of your game.
Go ahead and take some chances, is what I advise. Do the thing that scares you. The nice thing is, it's just a game, so that scary thing won't really hurt you. And maybe your players will even forgive you at some point.
I'm feeling ya InfoStorm. I'm doing the same thing with my group. I have new players and old players starting a human mine that was overrun by kobolds. Basic, but I have secrets and all sorts of things they are going to freak over. It's tough. I spent alot of time putting my head into "Kobold Mode" to really flesh things out. We are starting it this weekend. Hopefully, I'll give that sense of exploration and the unknown for my old and new players.
So, I think the major thing is going to be getting those creative juices flowing. Bruunwald, I think nailed it with the muses. Find things that interest you or rather what you think would be cool to go through.
Oh, another sidenote might be ask what your players might want to do. Having some expectations out in the open might help manage through it as well.
I'm designing a world from scratch, but I'm allowing the players help me design certain areas that have caught their interests. That has really got them pumped about exploring and keeping things fresh. Just have to keep them on a tight leash, so they don't go overboard.
I do most of these same things, if not all of them. My world is populated with dozens of custom monsters. But I have a lot more tricks up my sleeve. I will frequently take existing monsters, magic items and spells and reskin them so that experienced players don't recognize them. I bring in weather and environmental effects. I use crazy things like "magic storms" which have wild and unpredictable effects. I've got an entire area in my world that is basically stone age culture.
To keep even common things like a goblin lair fresh I will give a few goblins some class levels so they can do things the players wont expect.
Also, don't underestimate the power of new miniatures to get that sense of wonder flowing again.
Lately, with the computer and worldwide web, I try to bring in images or videos that I think will help players visualize a setting or situation.
Some DMs use music, but I've never done that.
I have also found over the years that it is almost always the humanoid NPCs that bring the players into the game. Give them a personality that the players can sink their teeth into. When your players say "Man, I really HATE that sheriff!!" You are probably doing something right...