There are really only three things the Rest XP applies to:
XP for Monster kills.
XP for crafting an item.
XP for gathering an item.

So, you have to one or more of those.

At higher levels (30 and above) much of your XP will come from other sources, so you will have to work hard to use up all of the Rest XP.
Doing mining/botanist leves or running dungeons close to your level are likely the most efficient means to burn through the Rest XP.

Personally I wouldn't bother about it. Having lots of Rest XP is not a problem, and to use up enough that it isn't "wasted" you will have to do some grinding.
If you don't actually enjoy doing said grinding, then don't do it.