Back in the old days (of B/X and BECM), we did it just a little bit differently. Like, if I were going to run an adventure module that was labeled "for characters Level 3-6," I wouldn't tell my players to "start your characters at 4th level." Instead, I would tell them "start your characters with 10,000 XP."
The PCs got experience for slaying monsters and earning gold, according to the rules. But I also awarded bonus XP for all sorts of other things, too: completing missions, exceptional roleplaying, bringing snacks to the gaming session...I thought every DM did that. (shrug)