Paw is hands down the best REST client for Mac. If you haven't already tried it I would highly recommend you checkout their free trial.
Multi Selector Dynamic Value is an extension I wrote to easily manage values in a list... There's really not much more to explain other than I have found it very useful when changing values for API calls without lots of extra typing.
Here is a screenshot:
The full source code is available at github.com/elliotchance/multiselector.