Extensions, APIs, SDKs
First time tried writing a Chrome browser extension. It was quite simple, basically followed this one walkthrough https://developer.chrome.com/extensions/getstarted and google'd a few more small things. It boils down to two parts: 1) register an extension and 2) develop HTML+CSS+JS page that gets rendered as extension's popup window. Overall a very pleasant and straightforward experience, but I saw that bringing React into the story is a tiny bit more involved, and because what I was building was supposed to be only a mock/demo, I didn't want to spend time on unnecessary things.
Last time that I had such a pleasant experience (and even better) as a third-party developer was with Airtable Blocks SDK. They have a CLI tool, state-of-the-art React usage and a very nice API, and ability to embed locally hosted component on a live website has surprised me, it was a first time for me to see it being done that way, and sure is nifty.
I think that investing development time in producing an SDK/API that has a low learning curve and gets you up to speed quickly is very valuable. Many developers with estimate the entirety of 3rd-party integration based on that initial experience, so giving them opportunity to see the results in mere minutes is extremely important for addoption and general emotion bound to integration with that specific service.