There are may ways to make easy bottle bombs. Ill tell you a few,
Try mixing caustic soda (drain cleaner) with water in a bottle and then add as much aluminium foil as u can and then screw on the cap.
Try mixing chlorine with alcohol or coca cola or milk and screw on the cap.
Or you could make the best one and mix some chlorine with car brake fluid and screw on the lid. this one blows up with a big bang and a nice flame unlike the others which jjust blow up with a bang. Just to let you no the 1st two blow up pretty quickly so just throw them and dont shake them but with the chlorine and brake fluid you can shake because it wont blow up for a few mins so if you want to blow up a letterbox u have plenty of time to run