Joi validation of array

trying to validate that an array has zero or more strings in one case and that it has zero or more objects in another , struggling with Joi docs :(

validate: {
    headers: Joi.object({
            'content-type': "application/vnd.api+json",
            accept: "application/vnd.api+json"
    }).options({ allowUnknown: true }),
    payload : Joi.object().keys({
        data : Joi.object().keys({
            type: Joi.any().allow('BY_TEMPLATE').required(),
            attributes: Joi.object({
                to : Joi.string().email().required(),
                templateId : Joi.string().required(),
                categories : Joi.array().items( //trying to validate here that each element is a string),
                variables : Joi.array({
                    //also trying to validate here that each element is an Object with one key and value
                })
            })
        }).required()
    })
}