I also loved Garrisons, i loved the interactions with them, in the early WOD days the invasions were fun too.
Now i mainly use them for tradeskills.
To the people saying Invasions are pointless as the gear will be replaced, this is very true, but if you use a Lvl100 boost you end up with lvl640 gear if i recall? you can replace most of this by invasion gear as its lvl700. But yes you will still replace it in Legion probably in the first zone.
I have been using the time while waiting for Legion to level Alts, level tradeskills, farm achievements and currently, farm mounts.
Now that we can solo Upto endgame Cata raids, i normally clear a lot of them for Transmog gear and mounts, the added gold also helps. Plus some of the raids i never saw, so its good fun to go in and just check out what the bosses looked like, obviously you pretty much 1 shot them all, but atleast you get to see the zone and see the models etc.
Also they nerfed invasion XP, but today they kinda hotfixed it, while the actual stage EXP has been reduced, they have increased the kill exp of the mobs, i have not tested it yet but its a good way to get exp for alts as the mobs all scale to your level, and the gear thats rewarded is priced at lvl100, so you can farm a decent bit of cash while doing it. As the invasions get more commonplace it will be easier to hop from zone to zone picking up exp, lob in dungeons between them and i bet you can get from 1 to 100 fairly quickly if you really went for it.