How to describe JSON data in a spec?

I would recommend my js-schema JavaScript library. The primary motivation behind it was the same what you describe in the question. It is a simple and easy to understand notation to describe JSON schemas (or specification, if you want).

An example schema described in JSON Schema:

{
  "type":"object",
  "properties":{
    "id":{
      "type":"number",
      "required":true
    },
    "name":{
      "type":"string",
      "required":true
    },
    "price":{
      "type": "number",
      "minimum":0,
      "required":true
    },
    "tags":{
      "type":"array",
      "items":{
        "type":"string"
      }
    }
  }
}

and the same schema description with js-schema:

{
  "id"    : Number,
  "name"  : String,
  "price" : Number.min(0),
  "?tags" : Array.of(String)
}

The library is able to validate object against schemas, generate random objects conforming to a given schema, and serialize/deserialize to/from JSON Schema.