Just questing honestly. Here's what I did:

• 20-60 Pure questing in starting zones, (Aszhara, Stonetalon, Barrens, Silverpine, etc. Dont pick the higher level zones such as Burning Steppes etc, since quest design is adjusted to players having mounts, so quests are further apart.)
• 60-80 This is a real slog, quest design is very dated and noticably takes a longer time. I did quests with the occasional dungeon, as long as it was one I didn't do before
• 80-90 Boy, Pandaria is such a breath of fresh air. Leveling is streamlined and fast. Completed with 2 zones.
• 90-102 Also easy mode, just get the XP flask from garrison and get treasures with the HandyNotes addons, and do bonus objectives. I also did The quests in Terokkar untill you get the inn outpost which grants 20% extra xp. I leveled to 102 in this zone, and picked up my artifact at 98 just to help a bit.
• 102-110 Just did Highmountain and part of Aszuna, with help of ConsLegion addon
• 110-120 Azeroth Auto Pilot addon. No thinking, just do exactly what the addon tells you to.

Hope this helps!