And it is far too easy to get locked into the mindset of “OK, we’re on our weekly lockout for Ragnaros and Ony, nothing to do now” instead of thinking about new things to occupy their time. All of these things can play into raiding, as well.
Leveling alts? That gives you more options when you are raiding later on. And if you’re leveling in a group, that makes it easier for some classes (like warriors) to break through certain bottlenecks. The Four Horseman fight in Naxx will be much easier if you have plenty of warriors, even if some of them are alts.
City raids and TM vs SS? Get on discord, and do it in a raid group. Practice your coordination and fight against a variety of threats. Do it as a guild, and it is something that you can practice regardless of raid lockouts.
AQ War effort? Lots of mats needed to unlock AQ when it eventually drops. Sure, you can’t work on the Scepter until later, but the list of supplies the factions need to drum up are a known quantity, and can be farmed now.
Argent Dawn? Higher you get that rep, the less expensive attuning to Naxx will be. Arcane Crystals currently run about 45-50 gold on my server’s AH, so the fewer of them you need per character to get into Naxx, the better.
BIS list? There’s plenty of items on the BIS lists that aren’t in raids, especially if you’re looking at P1 and P2 pre-raid gear. Working on those is something you can do while seeing whether you get lucky in the weekly raid.
Professions? Making consumables for yourself and the guild is a big deal for raiding, and can really drive down expenses. Some recipes can only be found in dungeons, if not raids, and some items can only be crafted inside dungeons. That’s more content for your guild to do between raids.