London trains

on such a short journey I'd be surprised of they are offered on that route.

And...taking the Great Western Railway from London Paddington to Windsor you can go contactless. You can just use your contactless credit card or ApplePay/GooglePay enabled phone to scan in and out at the turnstiles. No ticket purchase needed.

https://www.gwr.com/your-tickets/smart-tickets/contactless-payments

There are no advance fares on this route-

If you want to use contactless be aware of the disbenefits-
a) On Contactless you can't benefit from railcard discounts;
b) Contactless charges single fares only so would charge you 2 x £11.60 fares for this journey rather than a £12.80 off peak day return or a £16.50 Anytime day return on a paper ticket.

So you are way better off buying a paper ticket on the day.

Windsor is beyond the fare zones so is also beyond the cap zones (extended cap zones only go to Slough and Oyster zones, which end at West Drayton.
Under some, rare, circumstances it may be worth using the Slough contactless daily cap and buying a £3.20 day return from Slough to Windsor.

If you are travelling from somewhere beyond Paddington it may be worthwhile changing at West Drayton in each direction to benefit from the Zone 1 to 6 single fare, then a £5.60 return from West Drayton to Windsor (paper ticket- this fare is £5.50 each way so again don't do contactless). The Zone 1-6 can be done on contactless, but whether on Oyster or contactless you need to physically change trains at West Drayton to swipe in and out.

@ isn31c - Thanks for the added detail on the contactless payment method.

For myself I'm a solo traveler, going one way to Windsor from London for a couple of nights so looks like it will still work for me.

The contactless single fares from Paddington to Windsor & Eton Central are £10.70 peak and £6.40 off peak. So off-peak there is no difference from the paper ticket for a return. This is the case pretty much for all of the contactless fares along this line. Railcard discounts on contactless aren't here yet but will be eventually.