Konversi array to string codeigniter

ini gimana ya? mau insert data muncul gini

A PHP Error was encountered
Severity: Notice

Message: Array to string conversion

Filename: models/Model_data.php

Line Number: 13

Backtrace:

File: C:\xampp\htdocs\reminder_pkb\application\models\Model_data.php
Line: 13
Function: _error_handler

File: C:\xampp\htdocs\reminder_pkb\application\controllers\Data.php
Line: 17
Function: simpan

File: C:\xampp\htdocs\reminder_pkb\index.php
Line: 315
Function: require_once

Model_datanya gini

<?php

defined('BASEPATH') OR exit('No direct script access allowed');

class Model_data extends CI_Model {

	public function load_agenda(){
		$sql = $this->db->query("SELECT * FROM konten_agenda ORDER BY ID_Konten_Agenda desc");
		return $sql->result_array();
	}

	public function simpan($post){
		$nama = $this->db->$post['nama'];
		$tanggal = $this->db->$post['tanggal'];
		$tempat = $this->db->$post['tempat'];

		$sql = $this->db->query("INSERT INTO konten_agenda VALUES (NULL, $nama, $tanggal, $tempat)");
		if($sql)
			return true;
		return false;
	}

	public function hapus($id){
		$this->db->where('konten_agenda.ID_Konten_Agenda', $id);
			return $this->db->delete('konten_agenda');
	}
}