I only ride on the road away from home... (mountain bike happens in paddocks around house with my kids)

So when on the road...

I have my bike, my cycling computer/speedo, HR monitor and seat bag which has a spare tube and tyre lever amd gas cylinder in it.

My shorts, shoes, socks and jersey... jersey pockets have gas pump (too long to fit in my seat bag), a muesli bar and a cellphone.

My head has helmet, headband and sun/bug glasses.

Gloves of course are essential.

In winter you might need plastic bags inside your shoes to cover your toes (or buy neoprene booties - I use the plastic), arm warmers, leg or knee warmers, wind-proof vest and/or rain jacket, winter gloves.
Bib shorts are quite nice on cooler days as they keep the lower back covered and warm.