If you're at or above 80 Agility and cheap OSRS gold don't wish to buy Karamja's gloves 4, you can take a trip to Rellekka and begin training there. The Rellekka course isn't as competitive as Brimhaven Arena, however it's an acceptable option in terms speed and xp rate.
Although this one won't give you much better exp rate than Rellekka course - the levels of training in Agility can be boring so any type of variety is useful. It's a good course to go to If you're bored of Brimhaven and Rellekka and Brimhaven and aren't over 90 yet.
If you're above 50 in your Fishing skills and above 15 in Agility, there's another option to improve your skills that involves Barbarian Fishing. It is possible to catch fish that leap up like Trouts, Salmons and Sturgeons will reward those with previous experience in both Agility and Fishing which could be a great alternative to the traditional methods. If you're hoping to level up with this activity to 99 Fishing, you'll be able to gain 74 Agility through the process. Remember that 15 Agility only unlocks Leaping Trouts. For Salmons it is required to have 30 and 45 for Sturgeons.
To start Barbarian Fishing head to Otto's Grotto equipped with 15 Agility 15 Strength and the 58 levels of fishing. Talk to Otto and after a quick conversation explore under his bed where you will find a heavy Fishing Rod. Be sure you have enough feathers or bait in your stock and then you are able to begin Fishing. While you catch Leaping Trouts, Salmons and Sturgeons you will get some of the experience you need in Strength and Agility in addition to regular Fishing exp.
It is suggested to start this type of OSRS GP training as soon that you can as it is best for the lower levels of Agility and not as good for those near 99. Additionally if fishing is your second skill to max after Agility you'll be wasting many hours of Agility to nothing. Because Agility training can be dull doing Barbarian fishing is an excellent activity to consider.