I am a big fan of putting ice in the bladder, if it is available. Many times, when stopping for gas, they will let me get ice out of the soda machine without any charge once I explain what I want to do. Also, when finished with lunch, if we eat at a restaurant, I will ask for a pitcher of ice and dump it in the bladder.
Realistically though, I have not found that wearing my backpack makes me any hotter than not wearing it. I wear a KLIM Badlands Pro jacket. The armor is ribbed and also has holes in it to allow for air flow even if it is pressed against the back. Also, my backpack has raised pad with gaps between them that also allows for air flow between the pack and the jacket. The only real issue is how much weight I am willing to have on my back. I have found that I prefer to have a three liter bladder for a single day ride, especially if it is hot. There have been times where I have drained it by noon, or shortly thereafter! I sometimes even carry a ceramic water filter so I can refill it from a creek, lake, puddle, whatever. But, that is only for times when I am WAY out in the boonies.
One piece of advice I would stress is don't put anything in it that has sugar. If you forget to clean it right away, it will start growing all kinds of fuzzy things. This is what I currently use:
https://kriega.com/riderpacks/r20
The bladder opens at the top completely, so you can stick your entire hand inside it. It folds over and has a clip that slides over the fold so it cannot come open. It has NEVER leaked at all. It makes cleaning super easy compared to the Camel-Bak bladders with the screw type lids. Filling is also easier, especially with ice. The backpack can hold quite a bit. I usually put stuff like extra gloves, snacks, sweatshirt (when it is cold), and other light stuff so that the water is the only real weight. The backpack sits VERY comfortably on my shoulders and back. I don't really even notice it when riding. It is also quite durable. I have crash tested it with a nasty high side into a lot of dirt/mud. I landed on it and the bladder did not rupture and the pack itself shows no indication of wear. I have also worn it in heavy rains and the contents stayed dry even though it is technically not water proof. I still put things that MUST stay dry in big Zip-Loc bags.