So, we used to have the track at Regency and Hobby Habit used to have a track out the back but they''re both gone now.

The only current on-road track I'm aware of is in Littlehampton. Are people racing on-road anywhere else in Adelaide that I'm not aware of?

Or are people getting together for some fun on a tennis court or something?

Just at littlehampton that's it

They've all gone, except for Littlehampton.
I found that if take anything to a tennis court or something similar, first I get hounded by kids asking how fast and how much, and not long after it's the fun police because someone complained about the noise.

I've used the circled area in the Adelaide airport precinct many times on weekends. It's clean, really flat concrete, and grip is quite reasonable. Great for running motors, and just doing a tune up. I don't use the road, as traffic will come out of the blue, so stick to the apron area. It's about 60 metres long, so you can get up a good head of speed.
Sometimes I'll go there at night with electric cars and bikes. It's also away from prying eyes, so no kids hanging around asking how fast it goes, or how much it costs, and I have never been asked to move on from that area.

Been looking around for the same but no luck. Was thinking about bashing on the Vic Park track once the V8 race is all over. Keen to know where others are racing/bashing their on-road cars.

The spot you mention is federal land, so don't get caught there. I'd highly recommend not using that area in future. All airport is federal, even harbor town car park is federal.

I got caught years ago at Harbourtown with my 10th nitro. doing a little engine tuning. The security guard was very polite. I've used the other space a few times, probably 3 or 4 times a year. I'm never there longer than 15 minutes. That's probably why i get away with it.