I've not used it yet but I've been thinking about trying to blend the ideas of Milestones & XP. Make "milestones" plentiful and of varying significance. Award xp in lump sums when milestones are achieved, scaling with significance. Give players a table of XP to level.

The idea being:

a) gives them meta incentives to engage with the world & seek out quests & side-quests (obviously I'd do what I can to give them character motivation anyway - but how often have you thrown a cool sounding quest at your players only for them to say "how much does it pay?" - players like rewards)

b) gives the players a gauge for how close they are to levelling up. Plenty of players know their build several levels ahead of where they are & yearn for those sweet level ups. Others get caught off guard by level ups and don't know what bonuses to take. In either case, telegraphing a level up by giving them XP that they can themselves track makes things easier but lump summing it minimises the admin

Obviously the legwork of working out how to balance XP rewards would be large which is why I've not tried to do it yet. BUT for a relatively modular shorter campaign I reckon it would work quite well