Magic items found.
That is pretty much the ways to get expo in 1e by the book, as well as being in order of importance.
I find that expo awarding is one of the most house ruled parts of gaming. Me, I house rule it for pacing purposes.
In my once a month S&W delve into the Castle of the Mad Archmage, I find I need to award more expo than I would in a weekly game. My players want to explore, I went them to explore, and the only way to accomplish that is to award more expo so they can level a bit faster.
What I do with them is I offer a "room bonus" based on the number of rooms explored without taking a break to camp or re-memorize spells. The first room is worth 10xp for the party, the second 30xp, the third 60xp and so on. I cap it ay 1k per room, and dungeon levels that are not much of a challenge either grant reduced xp or none at all for exploring.
This has the potential of pushing my party to take risks. One more room could be an extra 1k xp in the party xp pool - it could also mean a TPK when they are low on HP and spells.
Does it get any better than this? ;)