I carry all the usual tools and tubes in a seat pack. Frame mounted pump.
Full set of clothes inc shoes. Shower gel, towel.
Laptop and charger, works phone, personal phone. Water bottle(s) as it's 25 miles each way.
We have non-allocated desks, and I work at home 3 days per week, so don't have any storage at work. Hence I carry the full shebang.
Am working on bagging a bit of filing space for stuff like towel, shower gel, shoes to save a bit of weight.
Oh, and leave a lock on the racks. It's not bagsying, just lock bike to available rack, so the lock moves around a bit.