Given that lunch in Windsor can range from a couple of quid to a couple of hundred, what sort of price range are you considering?
There's a couple of pavement cafés / ice cream parlours on Thames Street (the road down to Eton Bridge) which you can bring your bike to the table. From Eton Bridge, there's Brown's, on the corner of River Street and Barry Avenue, with a load of outdoor tables looking over the river.
Or, again, on the pavement at the bottom of Peascod Street down towards 51, there's a pizza place down there. There's also a few pavement cafés in King Edward Court, with Costa, Windsor Great Perk and Cinnamon in the Royal Station, you can leave your bike within view of any of those.
You can sit outside the Chicago Rock on William Street if you are looking for a bit more expensive or there's a couple of greasies, like Monty's, on St Leonards Road.
For something a little different, the Crooked House next to the Guild Hall, although there's no bike parking I think you can lock them to the railings round the Guild Hall.
I've saved the best to last though, as my favourite is the one along Barry Avenue (between the Royal Windsor Wheel and the Thames). Strange opening times, as it's more for tourists than travellers, but, if you are going for lunch, you can sit on the grass by the river.
*sigh* I wish it was summer again.
