For the MTB's I've got two 'camelbaks' - one for each bike. Old MTB has the smaller pack (decathlon) with waterproof, pump, CO2, 2 x 26" tubes, multi tool and a gel and a cereal bar. I just sling it on an off.
Newer MTB, same kit, except 27.5" tubes and a spare set of disc pads (depending upon weather - I've twice eaten through a set of rear pads in a ride). That rucksack takes a better 'bladder' (genuine camelbak). Occasionally I'll have a cafe lock in it too.
Road bike, I'll actually take one of the camelbaks as it saves re-packing, I just swap out the tubes. It's also a bit of a 'security feeling' for me having padding on my back (having badly broken my spine on a road bike 5 years ago) - I just feel more comfortable with something there, in case I fall on it (as I'm missing a chunk of bone).
I've got a couple of big boxes of gels - bought at a silly price. Handy just having them there in the rucksacks, just in case !