Have you played in an old module and had amazing fun? Of course you have! But did you go back again? What happened in the Village of Hommlet after you left? What came later to the Caves of Chaos after you had plundered it the first time?
I absolutely LOVE to re-use old modules; but not just in new ways that fit them into a new campaign world; but with the same characters that went through them before. Your powerful character is now a Lord or Ranger Lord; perhaps even an Arch-Mage...but what do they remember of their youth when they were scraping copper coins together for their treasures eh?
"As you sit and ponder the formula for the new spell that you feel will enable you to control the weather in your lands so that farmers will have an easier time of things and merchants will visit more often; you glance out of your window from your high tower and see a group of riders coming hard toward your tower..." Minutes later you are in an audience with a group of men from the Village of Hommlet.
"Great Wizard, we heard in our youth that you were one of the adventurers that defeated the monsters of the Caves of Chaos. Unfortunately, the caves have been lost to our guardianship as they were recently invaded by a powerful demon and his orcish host!"
Looks like its time to go back to the Caves of Chaos!
You see, you can re-use any adventure location when you are stuck for something to have your characters do! The wizard can grab a couple of friends and return to the Caves of Chaos to fight the demon and again cleanse the place of evil so that the Keep on the Borderlands and its new village can be safe. You already have the map, now you just need to quickly stock the place!
You see, the first time you went to the Keep on the Borderlands, you were just a new adventurer and things were really different. You had no idea really what was there...but this time you get to return like Gandalf going through Moria...you know the place, you have been there! In fact, you may have a few advantages knowing secret ways that the new inhabitants may not even know about...of course they may have set new traps; and obviously there are going to be new monsters as well. It is sort of thrilling for adventurers to go back to places and have knowledge of something. Also, the game play tends to go faster as the mapping is easier and the characters don't have to worry quite as much...especially if they kept their maps from before.
Something else to consider with old adventuring locales: use them again! Having a base of operations is a solid investment for players. While some people simply like to sit down and have good ol' hack-n-slash dungeon crawls, there is likely to be at least one player in your group that likes to keep their character's life "tidy" and know what is going on in the background. Take for instance a party that goes through the Village of Hommlet before taking on the Temple of Elemental Evil. What if they decide to STAY in Hommlet?
As a DM, it is sometimes a good idea to perhaps make it easy for your party to make a base somewhere. This allows you to develop that area and give it a depth that you can feel accomplished in making, and the party will really appreciate as they are able to more deeply immerse in the environment with their characters. Having a base (such as Hommlet which is already made for you!) has huge advantages. Your party may replace the corrupt merchants and start their own side business to invest their money in. They hire a Shop-Keeper to keep tabs on things while they are off adventuring; but when they return to town, they have a stable place to stay and can easily resupply too. Also, their hired henchman can make orders for them, attempting to acquire potions of healing from the local churches or from even far away Verbobonc, Dyvers or Greyhawk City.
Other party members may take it a step further once their reputation and power grows. Maybe the party Fighter decides that the old Moathouse really needs to be permanently garrisoned so that monsters don't keep creeping into the area and using it as a base...maybe you have already had the group go back to the moathouse because a group of bandits took up to living there again; or worse, maybe cultists started using it again. Either way, some of the Fighter's (or even Cleric's?) money might be used to rebuild the place and they might station their soldiers there!
One of the best things that can happen out of all this though: REVERSE DUNGEONS!
The party member(s) may have a base of operations...but do their enemies know about it? Do they have enemies that want revenge for wrongs? When the party has everything set up and feel secure, are they willing to have their base invaded by evil "adventurers" wanting to claim a powerful item of magic that now rests in a vault?
I've only been able to do Reverse Dungeons a couple of times, and both times they were great fun! They are best used as only one session surprises for the party member(s) though. Give them a bit of a scare, but don't overwhelm them and make it feel like they have no hope. They may lose resources, hirelings and even have a valued henchman slain...but that creates the opportunity for more adventure really as they seek to have their companion Raised from the Dead or as they track down the attackers in a new lair that you have designed. It is a wonderful way to spend an evening!
So, don't just go through a module once...don't just use the dungeon you created once. Remember them and think about what is happening in those places your party has visited in the past as time passes by. It can be a very valuable resource to Re-Adventure in Old Places!