I'm now back from my trip, and it was very enjoyable and well recommended.
Cologne was an interesting city. Not as pretty as Berlin or Munich and quite dirty, but still lots of interesting things to do, both with transport and tourist things. I enjoyed the Stadtbahn, as I haven't experienced a system like that before - where its like a cross between Metro, tram and S-Bahn. Particularly line 16, which goes all the way to Bonn, which I traveled on. Started off as a Metro in the city centre, then a tram, and then outside the city was more like an S-Bahn. The 16 route was excellent - with views of the Rhine, and then through the countryside, and it also goes right through the middle of a very industrial area at one point. However, I didn't appreciate the hard plastic seats for a 45min+ journey!! All the Koln Stadtbahn has the hard plastic seats, which are ok for short stops, but not for the many long journeys you can do. However, getting to know Cologne, I can imagine the padded seats might not last as long! The new electric buses on the 133 bus route also have hard plastic seating.
I tried out the new sections of lines 5 and 17, which seem very under-utlised. I started a journey at Heumarkt - I was the only person in the whole (huge) station, and then I had the whole train to myself until the Hbf. I'm guessing the intention is to connect these two lines in a North-South tunnel? I didn't see any evidence of construction work though.
The Hauptbahnhof was rather interesting, and a very busy station. You can still see evidence of the old station buildings. There is also great views of the station (and photo spots) by the Hohenzollern Bridge.
With touristy things, I thought the chocolate museum was excellent. I also went to the Zoo which had the best elephant enclosure i've seen in a zoo. I did try the cable car, but it was closed due to 'technical issues'. The Altstadt was good for a stroll, as was the walk along the Rhine (both apparently 'no go' areas in the evening, but I felt perfectly safe - there were bands playing music, and families all walking around).
I managed to catch the Class 110 loco-hauled journey on the RB48. Was certainly like going back in time. I got a few pictures, and I saw quite a few other people taking photos too, so presumably local enthusiasts. The coaches are also ancient (although seemingly with refurbished seats), with the windows capable of being opened halfway. Many passengers who are obviously used to the newer stock had difficulty with opening/closing the doors (as did I!). It was a great experience on that journey having a carriage to myself with the window half-open with old rolling stock. Also felt a bit odd being on a service run by National Express.
I got off at Solingen Hbf for the trolleybuses. I caught the 681 from the Hbf to the centre, for a few photos, and then from there the 683 up to Wuppertal Volingen (683 was an interesting route too). They were mostly modern, but with quite a few older 90s Van Hools running about I'm guessing the reason for the short section in Volingen where the trolleybuses operate under their own power is to do with clearance underneath the Schwebebahn?
At Wuppertal the Suspension Railway (schwebebahn) was a brilliant experience, and I felt like a kid in a sweet shop (or model shop!). It was bizarre seeing it running down the street (although most of it runs along the river - even more bizarre!). The service was frequent, and very busy - difficult to get a seat. While it is run as part of the public transport, it was obvious that a few people were travelling just for the experience like myself. As there are no tracks underneath, you get very good views. I travelled on both old and new types (and saw the preserved one running about - not sure if it was in service). The new types have excellent views at the rear with the full-length rear window. Wuppertal was quite scenic with the surrounding hills, and the old industrial buildings which appear to be from the industrial revolution era. The Hbf was under re-construction and appears to be nearing the final stages. It looks like it will be impressive. I then caught the train from here to Dusseldorf. I would recommend my itinerary for this day, if you like a mix of train and trolley. The RB48 to Solingen (with the BR110 depending on how long that will run for), then trolley 681+683 to Wuppertal.
Dusseldorf was a nice city, and I think better laid out than Cologne, and a more attractive city. The Dusseldorf Stadtbahn was very similar to the Cologne system. One thing that crossed my mind is what will the ageing rolling stock (both in Dusseldorf and Cologne) be eventually replaced with? While they are low-floor in the U-Bahn sections, most of the tram stops are not, and the folding steps come into use. Surely any replacement would need to be completely low-floor - which would presumably mean some very expensive alterations to many of the stops. Will be interesting to see what they would be replaced with, although they look like they've got a few years left in them yet. I didn't go on the actual tram network, as that was fairly unremarkable (to me) with all-modern tram types. I also saw some disused tram tracks in the Altstadt.
Bonn was a pretty city to walk around, and again the Stadtbahn is very similar to Cologne/Dusseldorf. I liked seeing the 1970s stations - they still look quite modern despite their age. I did travel down on the scenic 66 Stadtbahn line to the beautiful Konigswinter for the rack railway. This was much busier than anticipated, and many (like me) had to stand for the journey - that did take some enjoyment out of the journey. It was worth it for the 1955 vintage rolling stock, and the fantastic view of the Rhine.
The Hbf was simply awful, but it is in the middle of reconstruction, so i'll let them off. The majority of services through the station were badly delayed and over-crowded. I also had bad luck with the Stadtbahn which had a lot of delays and gaps in service. Perhaps I was just unlucky that day.
They do have a few Mercedes O405GN buses running around, which are now rare in public service - I did have a ride. I was also surprised that the only direct transport to Koln Bonn Airport was a half-hourly bus service.
For my Cologne/Solingen/Wuppertal/Dusseldorf trip, I did use the SchönerTagTicket for 30 Euros which was incredible value. A ticket which covers a whole state, and all its local transport, seems unheard of in the UK! It sometimes costs me that much for a single train ticket from Kent to London.
I would have liked to have spent a couple more days there, as this has probably been the most varied and interesting area i've been to in terms of transport.
Sadly there were no reasonably priced train tickets back to the UK, so I caught a £35 Flybe flight from Koln/Bonn Airport to 'London' Southend, where a rail replacement shuttle bus (operated by DB owned Arriva!) took me to Thorpe Bay for the c2c. As much as I like to promote going to Europe by rail, this was the most relaxing and stress-free journey by plane i've ever had (despite no trains from Southend Airport). Koln/Bonn airport was well run, and Southend Airport was excellent. It took me under 10 minutes from getting off the plane, to me getting on the replacement bus.
Thanks again for the recommendations - it really helped me to make the most of my time there.