Buy a saddle bag. I've got a Topeak Aero one, but many, many other makes are available*.
Into this put:
Decent multi-tool*
Two spare tubes
Set of decent tyre levers*
A pack of Scabs (or similar instant patches)*
A small spanner* (if you don't have QR wheels)
You should also have room to squeeze in a pair of disposable gloves to keep your hands clean.
On the bike, fix a decent mini-pump (I've got a Topeak Road Rocket*) - or you could go for CO2 instead.
You're now covered for 99% of eventualities, the remaining 1% of which can be covered off by a mobile phone and a credit / debit card which will easily fit in your pocket.
You can now ditch the rucksack.
* - there are nearly as many brands of these items to choose from as are there are opinions which are the best. I've got Topeak for most which I find are good quality at a sensible price. Some people like Lezyne, some Park Tools. To be fair, you won't go wrong with any of those three brands. Someone will be along shortly to recommend something different...
One final bit of advice, if it's been a while since you've fixed a flat spend a comfortable hour at home practicing what you'll need to do. Getting the process right in your head here will pay huge dividends out in the sticks...