Along the Thames, eat/drink/hang out on the riverside terraces of The Waterman and the Christopher Wren Hotel. If it's mizzling, there's inside tables at both, plus at The Cote in Eton, just across the little pedestrian bridge.
Eton's a charming walk-about - we just did the High St. one quiet August evening - may not be so quiet when school's in session. Great little quirky shops, bespoke tailors & Eton outfit suppliers, tiny pubs, the College edifices.
French Brothers' river tours - we enjoyed the two-hr; views of Eton's fields, posh homes, the lovely calm river, ducks & weeping willows, plus you go thru a small lock. The other marine traffic was all private vessels on our day, some quintessentially British- eccentric. They sell coffee etc onboard, for anti-jetlag caffeine, & narrate the route sufficiently to fend off sleep. There's open-air plastic seats upstairs, canvas-roofed, & cushier seats & tables along the downstairs' windows. Decent WC (with window!).
Walking - the paved Thames Walk in town parallels the river, & maybe continues straight out into the countryside - from the boat we saw dog walkers, fishermen, joggers, even a horse&rider along the banks. In town the main streets are full of souvenir shops & food joints, but if you go past the Castle (gawk at the lonnnnngg line of people who didn't pre-order their entrance tickets) (ORDER YOURS NOW if your trip's soon), there's the green expanse/straight-arrow Long Walk, with surrounding nice residential streets & gardens.
Food - we ate excellent fish&chips at the Horse & Groom, humble hole-in-the-wall pub near the castle; The Real Greek, modern building but great food if you like Greek; Bel & The Dragon, ye olde pub, pricier serious menu (great mussels). Didn't look for afternoon teas, but you can google.
Be aware that the Castle grounds have no refreshments for purchase, save one ice cream stand & the fancy chocolates in the several shops. BYO - you'll open your bag for security at the entrance, but they allow water bottles & snacks. I'd have paid extortionately for a tea or coffee mid-tour. The free audioguide is very good, ditto the info in Rick Steves' England bk.
Windsor's a great choice. Enjoy!