Just to illustrate some of the suggestions above. This is my bike on Saturday:
Note:
Oversize water bottle. Not sure how big it is, I got it from
Decathlon.
Saddle pack. This is an Alpkit saddle pack. It has my tools (incl CO2 inflators, and lots of other bits of kit), tubes, pump, some other bits and pieces and also my Goretex jacket in it. This one is expandable so without the jacket in it it squeezes down much smaller.
The saddle pack does partially obscure the light on the seatpost, but I also have Fibre Flares on the seat stays.
Spare room on the seat tube. I could put a bottle cage there. I have a Bikebuddy that fits on there so I can carry a thermos of coffee in winter.
You can ignore the handlebar bag. It's a cheap Decathlon job and quite the best, handiest bit of bike luggage ever. But not the subject of this thread.