The HTML page(s) should have two forms. The first is the form you will construct in the activity that connect to Open Weather Map, lets a user input a city or a zip code and asynchronously shows the weather information retrieved from Open Weather Map (via a GET).
The other should be a form that submits to http://httpbin.org/post.
This from should submit asynchronously via a POST. It needs to send a content-type of application/json (you can also experiment with other content-types like application/x-www-form-urlencoded). You should display the data you get back (which should match the data you send). It will be stored as a string in the data field of the JSON encoded string returned from the server.