Getting in with alts
Alt raid runs are a way to keep players engaged, although it can be a little bit more tricky to get people engaged in them. Some players are massive altoholics (myself being one of them), and there's a large number of people that at least have one or two alts at max level that they enjoy playing. Running raids, even current content, with alts is a great way to pass the time once your main raiding group has already cleared everything. It may seem like a lesson in insanity, but that isn't entirely the case at all. Encourage players to bring in alts for roles that they don't traditionally play. Ranged DPS can be healers, tanks, or even melee DPS, as an example.
Doing this widens a player's horizons as to what other raid members face with every encounter. When content becomes difficult, it is rather easy to fall into that rut -- the notion that your particular role is the difficult one, what others have to deal with is so easy, and you just can't understand why they aren't able to do it. By switching up roles with alts, you give players the opportunity to put their mental money where their mouth is and prove just how easy the other side is. Often, this leaves them rather surprised.
You don't have to run current content with alts, either; the prior level of raiding content is just as good. Anything that actually challenges the player is a good choice. Remember, it's all about fun. The game itself is about fun, but difficulty is a part of that fun. When everything is too easy, then it can be difficult to focus, engage, or even care about what's going on. Sometimes that's good, and sometimes that's bad. A nice mix of both is the best policy.
Oldies but goodies
Running old raids isn't a bad option either, and it is one that can be done with alts or mains. There are tons of older raids out there, all of which are still loads of fun even if they don't present any challenges. Going back into Wrath, there are still achievements to earn from Icecrown Citadel and Ulduar that can reward players with mounts. Getting every achievement is a single run usually isn't possible; some of them take a long time and some of them are contradictory, but there's never any harm in shooting for them.
Many players have also never gotten the chance to see some of the great raids out there such as Serpentshrine Cavern, Tempest Keep, and, yes, The Battle for Mount Hyjal. When the current raiding content is done, then going back to see many of these old raids offers a nice change of pace for players to enjoy. There's little stress involved, the content is sometimes new and refreshing, and it's a nice break to take advantage of.
Along with getting some old achievements, which come with mounts or titles, there is also the prospect of gaining items for transmogrification. With the upcoming patch, there are tons of people clamoring to get their hands on older pieces of loot for the sake of playing dress-up. Since you are going into old raids, you don't all have to go in as one large group. While that does add to the fun and community, you might want to break into smaller groups so that more players have the chance to get the loot they want.
Adding in a little spice
More than just these more basic forms of entertainment, you can always spice things up as much as possible. After all, one can only run content so many times before it begins to feel flat. Finding ways of making old, easier content more enjoyable is an effort all on its on. Again, challenging players in the best way to get them engaged with the game.
There are simple ways of creating such things. You can break a larger raid group into two smaller groups, then have them race against each other to clear a particular raid. You can cut down on your number of healers to push them all as hard as you can; even with better gear, managing to solo heal all of Blackwing Lair is quite a feat that a player can feel proud of.
You could also try other unintended means of increasing difficulty. Running tier 11 raids set to 25-man with only 10 players in one way to increase the difficulty, even if the content still isn't that challenging by itself. Some encounters, such as heroic Cho'gall, Al'akir, or Sinestra, can still remain hard even for players decked out in full tier 12 gear. Going back to do these encounters for the titles or achievements that they offer is still a great way to pass the time once you've completed all that the current content has to offer.
A change of venue
Another choice that you have is to engage in activities that your guild might not normally consider. Raiding enemy capitals is always a great option, and you can even split a raid, sending half to one city and half to another and see who can take out their primary target first. What is great about more free-form and flexible raids of this nature is that anyone in your guild or guilds can participate; you aren't restricted by numbers, as more traditional raids would be.
If you do only have a small, set number of players that would like to participate, then running Rated Battlegrounds can offer another challenge for your raiding group, even if it is just a fun side project to engage in. Not everything has to offer a tangible reward of power; most of it can be done purely for the enjoyment of the activity itself. I personally am not a huge PVP fan -- it just isn't an aspect of the game that appeals to me -- but occasionally getting involved in a fun round with friends holds some value. It isn't about winning or getting anything done; it's about the fun of playing with friends.
The clinking sound of gold
Should your guild be of a more lucrative mindset, there is also the prospects of organizing GDKP runs, raids wherein players pay for items with money. Even for a casual 10-man raiding group, you can easily split into two different groups, find five or so puggers per group, and run an older raid in order to turn in a decent profit. Tier 11 content doesn't offer the best of money and certainly isn't as lucrative as the old ICC 10-man runs were, but they wouldn't be a total bust. The additional money is just a bonus -- again the primary purpose is just to have an enjoyable night off.
Content is fun, but what makes it fun differs from person to person. Some want the thrill of a challenge, while some like to just spend an easy night cruising through content with friends.
Ready Check shares all the strategies and inside information you need to take your raiding to the next level. Be sure to look up our strategy guides to Cataclysm's 5-man instances, and for more healer-centric advice, visit Raid Rx.