As a local, I agree with the recommendations of a gentle stroll around Windsor, near the castle and around the olde worlde shoppes. If you're lucky, there may be a bit of pageantry too with horses and guards and stuff. Sometimes, HM zooms up the through the park in her jag and sometimes Phil can be seen wandering around in his racing carriage.
And the bus makes sense from T5.

Optionally, also take a taxi from Windsor to Bray (15-20mins) and have a 1230-1415 lunch at The Wateride Inn (3 michelin stars, lovely view of the river, excellent food and service) or the Fat Duck (also 3 stars but not so excellent imho and not on the river). Be sure to make a reservation.
15 mins back to T5 from Bray in time to "fast-track" slowly before boarding.