In completing each Achievement in RS gold each of the difficulties in the Achievement Diary players gain access to the exclusive Achievement Diary Cape, which they can purchase at Twiggy O'Korn in Draynor Village for 99,000 coins. Furthermore they'll also be rewarded with a unique hood that matches the cape.
Duel Arena (Safe). As the name suggests, Duel Arena allows players compete against one opponents. The place also offers rules specific to the game that can be set to bring the fun (like the staking of gold coins). The site is open to both players as well as free adventurers.
Clan Wars (Safe). It allows players from two opposing chats to engage in a battle against one another. There are no rewards for cheap OSRS GP this kind of activity... maybe besides glory.