We do not offer a sandbox right now, but you can use the following suggestions for testing our API.
Use "Validate : True" when placing orders via the "AddOrder"-endpoint
When placing orders (via the AddOrder endpoint) Use Validate : True so that it simulates the trade - it will return the parameters of your trade but without an Order ID (real orders that succeed will always have an order ID returned).
Use values for price that are far away from current market conditions
Sometimes you still need to work with real order IDs so you know that your order logic is working. To do this, we recommend creating limit orders with ridiculously low parameters such as Buy 1XBT @ $5USD or Sell 1ETH @ $1000000USD. You'll be able to know that your real orders have been placed, but they are likely to never be matched.
The rest of the methods are harmless in comparison - you might possibly make a mistake and run over the API calls limit - but that will only cause a temporary ban (usually lasts 15 minutes). Make sure that for public calls you do not exceed more than 1 call per second. Private methods such as querying trades and balances are limited by a cooldown counter based on your verification tier - as time elapses the cool-down counter will recover so you can continue to make API calls. Placing and cancelling orders does not count against the Rate limiting but please do not abuse it as we aware of nefarious activity and will disable access if such actions are suspected.