Pricklypear
Pricklypears can be pretty common and luckily have delicious red fruits! You can harvest them with your raw hands but those glochids are no joke, using shears might be in order... probably don't want to touch it either.
Not all cacti have red fruit
...Some cacti have red flowers instead! The fruit color will be different, but sometimes it's green, so the fruit will blend in a bit.
Tasty berries
Some smaller cacti have similarly smaller berry-like fruits. They aren't more than a light snack on their own, but I wonder what they might go well with...
Dry fruits
Sadly, not all cacti fruit were meant to be eaten. If the fruit is tan or brown, it's likely a dry fruit and is little more than a dry carrier for seeds. Wait for the fruit to dehisce and the seeds will pop out on their own.
Cactus babies!
Craft seeds from the fruits you harvest to get cactus seeds you can use to make your own garden. Just be sure to have patience, a lot of cacti are slow growers...
Padding
Pricklypears don't grow well from seeds, so the seeds from these plants are mostly useless. Shear off a pad instead and stick it directly into the ground, it'll grow on its own!
Ghosts of the desert
Some cacti are quite small and quite rare, making it hard to find many species. Most biomes where cacti spawn have a biome exclusive rare cactus. I wonder what would happen if you collected them all...
Treat time
Use your harvested fruit to make some delicious treats. I bet some cactus berries would go great in a muffin mix!