How to Get XP
You almost can't avoid getting XP- almost everything Bayek does generates some amount. But some means are more effective than others. Probably the most effective is Reda's daily quest, once you reach Alexandria. Reda quests grant side-quest level XP rewards for simple, single-objective missions, plus some high class gear- it's a no-lose scenario.
Another lucrative way is locations. Be sure to wander through any nearby question marks on the map, especially in cities. Not only do you get a small XP bonus just for discovering the new location , but you can usually get a couple hundred XPs just for scanning the site and looting a treasure box.
The most consistent way to get XP, though, is always going to be sidequests. While some are longer than others (much, much longer, at times), you're always guaranteed at least four digits in XP, plus the locations found and enemies slain along the way.
As XP goes, slaughtering enemies isn't a great strategy. It's a nice icing on your side-quest cake, but even with Ability bonuses, you'll rarely make more than 30XP per kill, and often less than 10XP.
Lastly, after completing the main quest, you'll have the opportunity to buy XP for drachmas on pretty much a 1-to-1 basis. On the other hand, after completing the main quest, you'll probably have a ton of XP anyway.