Berlin

SVts

I'm thinking of using the Hop On/Off bus tour.

Looks good value for £23.

https://www.berlin-city-tour.de/tickets/hop-on-hop-off-classic-tour/

Costingbunny

I was there a month ago for the first time for a long weekend and loved it.  Highly recommend a free walking tour with Sandalmans - please tip though!
train from airport is easy and takes 45-60mins depending on your hotel.  We stayed at About Berlin hotel which was a good location for bars, restaurants and near 2 metros.
Didn’t do the HOHO bus as you can pretty do this on the regular buses for a ticket price (Google it) but we walked a lot and saw things on our travels

stoneman

You will be going to Brandenburg airport. Flights from the UK go to Terminal 2, and every time I have gone it's taken at least 45 minutes to clear immigration. 
The train into the centre goes from T1 which is a short walk from T2. Really easy to get, better and cheaper than a taxi. 

Lost_in_translation

Airport to station (underground) is a short walk. When buying a ticket to/from the airport make sure you get Zone C. It doesn't happen always but they do check and it's costly (EUR 60, I think). Potsdam is also in Zone C and worth an outing.

Depending on how long you are there you could get a BVG (local transport) pass. You can get a day pass, 2-4 day tourist passes which include various reductions on admission prices, and a week pass. If you use make at least 3 trips each day then I think this will save on buying one for each trip. BVG includes local rail (including DB within the city), underground, tram, bus and a handful of ferries which cross the beautiful lakes (worth an outing?).

I haven't done a tourist bus, I believe they are good and give you lots of info. If you are being frugal however, the 100 & 200 buses form a loop around the historic centre between Zoo and Alexanderplatz. There's a little display board which shows the next three stops onboard so you can keep some sort of track that way.
If going in summer, make sure you get a hotel with air con, because Berlin can get very hot (over 35deg c) and traps the heat so it doesn't cool below 20 at night. It also has fantastic thunderstorms.
Final tip: when in Germany do not stream copyright material, because the lawyers are hot on chasing this up and send very nasty bills (nearly 1K!), and your internet provider will pass on your details. I didn't, but I know someone who did.
I actually came on here to post this final tip in the holiday habits to avoid thread (from the newsletter) but I can't find it, so if anyone can direct me...