Marlow
Probably the most scenic of the Thames Valley branches is the Marlow branch; nice views of hills as you run near the river between Bourne End & Marlow. The line used to continue from Bourne End to High Wycombe.
Trains from Maidenhead to Marlow reverse at Bourne End with the guard changing the points and obtaining a "one train only" staff which he gives to the driver as authority to access the branch. Bourne End has two platforms, and on M-F rush hours has through services to/from Paddington, with the branch train locked in and just shuttling to & from Marlow.
The station at Marlow is not at its original location, as the branch was cut back to the edge of town. Thus it's a longer walk to town than formerly, but it's a pleasant town when you get there with river bridge, attractive high street lined with cafes & pubs.
Henley on Thames
The Henley branch is less interesting operationally, being single tack with two intermediate stations. You cross the River Thames between Wargrave & Shiplake, and cross from Berkshire to Oxfordshire.
In the 60s it was double throughout; you might spot the platform at Shiplake which used to be an island platform (only the former upside is now used). The infamous open crossing at Shiplake, has now been fitted with half-barriers. The Baskeville Arms pub beside Shiplake station is a good place to get off for quality food and local real ale at lunch time, and because of the frequency of the timetable, you'll have an hour to eat and drink, which sound civilised!
The terminus at Henley on Thames has also been cut back slightly, the existing canopy being original, but this was just the part of the platform outside of the station's overall roof; the station entrance opened out on to the main road, while now it is one block back from the main road. Again, a pleasant town, lots of pubs and cafes.
Windsor & Eton Central
The shortest of the branches, at one time there was a triangle at the Slough end, so direct running towards Reading was possible without reversal. After leaving Slough, trains run non stop to Windsor, but I believe at one time there was a small intermediate Halt.
The remaining platform at Windsor is a shadow of the former multi-platformed terminus, but the canopies and buildings are all still there, and are full of shops, restaurants, and people. Windsor Castle is literally just across the road, and the station is right in the centre of this town which is full of tourists.
The town of Eton -and South West Train's Riverside station- is five minute's walk away: left and down the hill beneath the castle walls. Eton is on the other side of the River Thames over the wide bridge.
Greenford
I don't know a lot about the Greenford branch, except to say it's double track with a triangular junction at each end. There are no booked passenger services over the curve to Hanwell at the southern end, or over the easterly curve at the northern end AFAIK.
At Greenford, the Turbos run in to a bay platform at the London Underground station. Scenery is mainly urban, but with green playing fields etc.
Although services run to/from Paddington at present, once Crossrail is open, all the Relief Line (i.e. slow line) paths will be required for their trains, so trains from Greenford will terminate at West Ealing.
There has been some speculation about what will happen in the future; the line is not due to be electrified unlike the other three. Perhaps it could be transfered to Chiltern Railways; or perhaps it might be operated by London Overground, although they would surely want to electrify it. All conjecture, of course.
Other things to look out while travelling on a Thames Valley Rover, might include: work at the former Eurostar depot and North Pole to convert it for IEP trains; the new diver under being constructed at Acton Yard; the odd steam engine on Southall shed; extension to the flyover at Airport Junction to enable Crossrail trains to access Heathrow Airport from the Relief Lines; and the march of new electrification masts towards Reading.
Lots to see and do: have fun!