Final Fantasy X has quite a number of interesting sidequests to do, as is normal for a Final Fantasy game. Some of these are quite simple, while some border on the impossible ... ok, ok, they're not that hard, really, just quite tedious. I'll try to cover as many of them as I can in this section.
Al Bhed Primers
Tells you the locations of all 26 volumes of the Al Bhed language primers so you can finally understand what on Earth, um, Spira Rikku's tribespeople are talking about.
Exactly how to get the legendary weapons for all seven characters as well as how to power them up. Butt-kicking time!
How to access all the secret areas. Airship not provided.
Not much here, basically just tell you what you get for fighting (and winning) those battles.
The Monster Arena
Everything about capturing monsters from all over Spira so you can get some VERY valuable items as well as attempt to fight the toughest monsters in the game.
Chocobo Training & Racing
How to get these cute cuddly creatures to do your bidding so you can get some very important and useful items.
Rikku's favourite pastime! It's amazing how Tidus can't be killed by lightning ... Actually there isn't a lot here that I can help you with.
How to get the right ones and get away from the wrong ones. This is bound to be very frustrating.
These tiny, adorable and very scary monsters, surprisingly, actually wants to play with you.
How to collect the spheres left by Jecht, Braska and Auron. Interesting story to be seen from them, and necessary for getting Auron's Overdrive skills.
Information on getting the three secret (and most powerful) aeons in the game. You need to get all of them in order to obtain the Moon Sigil for Yuna's Nirvana.