If you've just staked your SNX to mint sUSD, there are a variety of things you can do with it.
Firstly, there is Synthetix.Exchange, which is a trading platform allowing you to trade your sUSD for any of our synthetic assets (Synths). It currently supports a variety of forex currencies (e.g. sEUR), cryptocurrencies (e.g. sBTC), and assets (e.g. gold/ounce as sXAU). The best thing is, it doesn't require a counterparty, so there's no waiting around for your trade to get filled. So for example, if you'd like exposure to Bitcoin without leaving Ethereum, you can trade your sUSD for sBTC on Synthetix.Exchange. All trades include a 0.30% trading fee, which are distributed to SNX holders as reward for staking the system.
Secondly, you can take your sUSD to UniSwap. UniSwap allows you to add your sUSD to the liquidity pool there, or you can use the pool there to trade your sUSD for ETH.
Thirdly, you can deposit your sUSD into the Depot through Mintr. This will put it into a queue to be sold at Synthetix.Exchange. When your sUSD is purchased, you’ll receive the proceeds of the sale in ETH. While your sUSD is in the queue waiting to be sold, you can withdraw your sUSD back into your wallet at any time through the 'Depot' tab on Mintr.