If you're above 80 Agility and don't desire to OSRS Gold purchase the Karamja gloves 4 you can visit the Rellekka and start training there. The Rellekka course isn't as competitive as Brimhaven Arena, but is decent in terms of speed and xp rate.
This course will not give you the same rate of return than the Rellekka course - the levels of training in Agility are really boring so every sort of variation comes in handy. This is a great course to attend in case you're bored by each of Rellekka and Brimhaven and you aren't past 90 yet.
If you are above 50 in your Fishing skills and above 15 in Agility there is another option to improve your skills that includes Barbarian Fishing. Fangs that leap, such as Trouts, Salmons and Sturgeons provide players with the experience of both Fishing as well as Agility. It is an alternative to regular methods. If you're hoping to increase your skill level by this method to 99 Fishing you will also be able to gain 74 Agility through the process. Remember that 15 Agility only unlocks Leaping Trouts. For Salmons you'll need 30, and 45 to unlock Sturgeons.
To start Barbarian Fishing head to Otto's Grotto with 15 Agility, 15 Strength, and the 58 levels of fishing. Talk to Otto and , after a brief conversation, explore under his bed where you will find a heavy Fishing Rod. Make sure that you have enough feathers or bait in your arsenal so that you can begin fishing. While catching Leaping Trouts, Salmons and Sturgeons you'll acquire some experience on Agility and Strength addition to the regular fishing experience.
It is suggested to start the training process as soon as you can since it's excellent for beginners and not as beneficial for those that are close to 99. Furthermore, if fishing happens to be your second choice after Agility it is a waste of the time and Cheap RS3 Gold effort you have spent in Agility for nothing. Since Agility classes are usually boring, Barbarian fishing is a fantastic option to think about.