Minecraft wiki entry for Experience_farming, associated text: Kelp gives little experience (6.4 XP/stack), but the smelted kelp can be turned into fuel for the smelter, with a surplus for other uses.
Potatoes give more experience (22.4 XP/stack); the cooked potatoes make a solid mid-tier food supply, and they can be composted for more bonemeal than the raw potatoes.
Chickens are also an option, as they can be automatically bred and killed to produce a stream of chicken meat, but for any given number of chickens, more experience can be had by manually slaughtering them. That said, having large numbers of chicks around waiting to grow up can also cause a lot of lag for the game, while a continuously running farm (which kills but does not cook the chickens) can get by with fewer chickens alive at any one time.
Cactus gives far more experience (64 XP/stack), but unlike the others it cannot be cooked in a smoker, and the resulting dye has no use beyond making things green. For a large-scale smelting setup, arrangements must be made to dispose of the dye.