I just discovered Luna Display this morning, which will allow you to use your iPad as a wireless second screen for your Mac. There are software solutions that do this, such as Duet Display (requires a cable between your devices), which I use regularly when I'm away from home or even working from a coffee shop. The difference with Luna Display is that it will access your Mac's GPU (assuming you have a Mac with a GPU) for better performance. It uses a dongle that will plug into a USB-C port on newer Macs and a mini-display port on older Macs.
Here's the promo video
Here's the Kickstarter page. They are well beyond their goal.
I use Duet Display for a second screen so often that I went ahead and backed the Kickstarter campaign as this looks as if it will be a much better solution. The downside is that it won't ship until May, 2018.