My advice is to mix it up a little.

If you're leveling multiple groups, definitely take advantage of rested.
Play a group until its gone or almost gone, and then move on to the next.
Depending on your play time, you might be gradually leveling two, three or more groups, before the first group reaches maximum rested again.

Questing is a fast option, but only if you efficiently stack the quests.
There are enough areas in the game, you can level two teams with minimum overlap - if you're willing to do collections as you get them.

Instancing can be a lot of fun, but the 33rd time you run Nexus or Utgarde might not feel as unique.
Still, there are a lot of instances; if your teams are fresh to 60, you could stay in BC dungeons on a team to 71 or 72 maybe.

Farming is generally the most boring method, but combined with a movie or a season of <insert show>, you're more watching the show then farming.
Pick an area with herbs, ore and things to skin, and play the team with 90% of your concentration on whatever you're watching.
Mash your assist/dps button for a bit, then loot/skin etc and move to a new area.

As soon as you're tired of whatever you're doing, switch it up to a different method or different team.
And if you're tired of the game in general, take a break, it'll be there when you come back.
Don't want to play until you're burnt out.