AFK fishing is about the simplest. XP and lots of enchanted bows, rods, and books. Helps accumulate spell books to get all your equiptment up to goal levels. Lots of trash too, and fish. Cooked salmon is a good food ompared to all except the meats.
I'm not a fan of AFK fishing, but as GeoFor says it's a very simple way to get xp while gathering some potentially good stuff along the way.
If you want something faster and more active though, an easy farm I like to make is a 2x6 hole surrounded by trap doors in the nether. All you have to do is wall off the back and make a way to get down to the landing area.
After that herding Zombie pigmen is easy enough and you can earn some good xp quickly.
Another reason to make a zombie pig farm is that you can gather gold to use in the villager curing process.
Once their prices drop low enough you can buy bookcases from librarians at the cost 1 emerald and then sell back the books. You can generate some ridiculous amounts of xp this way.
Villager trading is how I repair my mending tools.
If you do build a zombie pig farm make sure to place lower slabs around the killing area so they do not spawn on it.