I find it amusing how Garrison Resources were so coveted at the beginning of the expansion, and now I simply can't spend them fast enough. Patch 6.1 is changing this with several new resource sinks. We've already talked about rush orders and follower reroll items which can be purchased with resources, and next on the list is Scouting Missives.
You can buy Scouting Missives from your garrison quartermaster at a cost of 200 Garrison Resources each. Scouting Missives unlock one of the daily quests that reward 800 Apexis Crystals, e.g. Scouting Missive: Darktide Roost. These are separate from your regular daily quest, so you can do as many of them as you want as long as you have the resources to spend. You can see the full list of quests here.
If you're using any of the Apexis Crystal gear this should help you fast track the latest upgrades (ilvl 670 in patch 6.1). I'll probably use them to complete the Securing Draenor achievement.