This past June, I enjoyed staying one night at The George Inn, just over the pedestrian bridge on the Eton side of the river. Some of the rooms have a view of the castle. Perfectly comfortable, not fancy, with cooked-to-order breakfast included in the rate. Staff were very helpful (lent me a phone charger, carried my bag up the narrow stairs to my room, arranged a car service to Heathrow for ~17 British pounds).
Once I saw how busy Windsor was, I was glad to be on the quieter side! Easy walk to the train station, the castle, the theatre, etc. Also, Eton's main street had some interesting shops and an excellent little grocery store (Budgens) with a post office counter inside and very reasonably priced food treats to bring home for family and friends. I can also recommend the prix fixe supper at Cote Brasserie, riverside at the bridge. I hope you enjoy your visit.