FFS, a synth shop is a synth shop, so what, especially if you only have a few days - just enjoy what Berlin has to offer which is a LOT … and a lot of history - you gotta venture east at some point IMHO, might be a little more gentrified since I enjoyed a visit many years ago - great city
Just Music is huge and excellent. I go in there often, just to browse and to play with stuff. Their Elektron selection is not great; they still have an RYTM mk1 display stand! But you can certainly get the MKII stuff from them. They’ve got headphones you can use, but I suggest you bring your own, because the loan ones are, without exception, awful
Yeah spot on - it would be too obvious go to Berghain though I’ve never been - is it worth it just for the venue ? I’d rather have a mature night out with some deep classic vibes if such a thing exists anymore
Renate is a cool club. There are loads of nice eating and drinking options along Weserstraße and Simon-Dach Straße, tho the latter is a little more packed with tourists (one of my personal favourites is the stupid awesome Paule’s Metal Eck; it has plastic skeletons and other horror models stuck to the walls, heavy metal, and darts. Need I say more?) Vor Wien is a great bar with a secret tiny dance fooor underneath the bar (and looks a bit like your photo!).
Walther Bar, Wrangelstr. 83 is a very small and cozy bar good to visit at 9-10pm to get started having a beer while there‘s elektronic music being played on Fridays and Saturdays. Used to play there myself some time ago and it‘s a well balanced mix of original Berliners and folks that find it by accident. Berghain is awesome (even though it‘s not for everyone), but even the most critical ones found it to be great. There‘s always a chance though that you‘re waiting an hour and the door keeper is saying no (there are books about how to higher chancer to get in). Just don‘t dress like on a business trip, or go there totally wasted.
Watergate club is a little more moderate and probably a great fit as wel to go with your gf. If this is still not moderate enough go to Weekend Club (16th floor or so with view across the city, but very touristic).
Apart from that, Kosmonaut and Suicid Circus are great as well. Download the RA Guide app and you can find all major electronic events in the city.