Camelback "HAWG" has worked well for me but there may be newer/better/smaller options.

Get rid of the waist strap but the sternum strap helps. Trashbag inside if you need/want to keep clothes and shoes dry but make sure that you squeeze as much air out as possible because if it's too buoyant and there is a lot of wind or chop it will be hitting the backpack as you swim. It will add drag though.
Have fun!