To answer the OP, I suggest just questing, it goes by pretty quick, even while doing most collection quests.

I wouldn't just stop questing in Outlands just because you hit 68 and can start in Northrend. There's still plenty of fast quests in BEM, Shadowmoon Valley, and Netherstorm that you get 10-12k xps for (Northrend starts around 15k per quest). My preference is to exhaust Outlands before moving on, though you could do a dozen or so starter quests in Northrend for the weapon upgrades and a few other items before heading back to Outlands.

Exhausting Outlands, I usually end up somewhere around level 72.5 with only those dozen or so quests already done in Northrend. It slows my progression down a bit, but I tend to hit 80 with about 3.5-4 zones untouched, just waiting to be harvested for the gold boost.

People argue both ways, but it comes down to how you want to proceed. If you want faster levels, head to Northrend earlier. If you want to reap more gold at the end, then hang in Outlands longer.

Side note: You may have to go back to some Outlands quests anyways if you even care about achievements and events. But that had no influence on why I chose that route.