I would like to summarize some parts from my trip to Cagliari that ends tomorrow, mostly In favor of future tourists to come. Bare In mind that I don't speak Italian, and I believe some issues that may look ridiculous to Italian might actually pose a problem to foreigners, as happened to me. So here goes:
Buses - I chose Cagliari since I don't hold a driver's license and I got the impression it was the best spot in Sardinia for people like me. On the second day in town I got a 53 Euros fine at the CTM bus. This happened 30 minutes after I bought a weekly CTM bus ticket, but I haven't figured out how to validate it. I've shown the inspectors the date and the hour on my ticket, but they couldn't care less and fined me. I still didn't understand how to validate the ticket. The day afterwards I went to the tourist information center and there was a nice lady over there that explained it to me, said that CTM inspectors act this way a lot with tourists, and suggested I'll file a complaint (which I did). The day afterwards I rented a bicycle and it made my life a lot easier - buses inside cagliari tend to arrive late and be rather slow. So learn to validate your bus ticket, and rent a bike.
Poetto and Calamosca - I know there's an impression that the first part of Poetto is more commercialized and the far part is more secluded. Actually throughout all Poetto there are bars and restaurants that rent a beach chair for 20-25 Euros for a day, no part is actually secluded in the original sense. But it's true that at the beginning it's more aggressive. Calamosca is a small sweet spot right before Poetto with clear water and the beach isn't full of beach chairs owned by restaurants - I highly recommend going there (not in the weekend).
I admit that after the CTM incident I got a bit depressed (I was also alone all these days) and got a feeling that taking a bus to other beaches isn't worth it because the buses will be slow and the beaches full of restaurants and umbrellas. I'll be happy to be proved wrong.
Molentargius Park - I took the minibus tour that takes you to the protected areas where you see flamingos nesting - honestly I think it was the best part of my trip, entering the protected areas and then seeing them up close through the telescope. It was surreal and I highly recommend that
Boat tours - I was wondering at the beginning what's the problem going with an inflatable boat. Well. I booked a boat tour on a dinghy, unfortunately half an hour through the tour I really had to pee and didn't manage to pee in the water. I knew there were 2.5 tour hours ahead of me. I told the skipper who was quite cold and said a few times there's nothing to do. After a while I asked again and he told me he can let me off at the dock of Poetto but then the tour will end for me and I have to take my stuff and go. I agreed and so happened (I later confronted the operator but mostly about the skipper's attitude).
So that's why you might want to go on a boat that has a toilet.
Besides that the tour goes in 3-4 places very close to Cagliari and stops in each for 20-30 minutes and people can go swimming or snorkeling. The water is clear but all you see is sand and small grey fish.
Being vegan, gluten free and trying to eat healthy in Cagliari - I know, I know.
Palazzo Regio - I didn't see that coming but that's an awesome well preserved palace, which was renovated in the 18th century mostly. It's quite impressive and the entrance costs only 3 Euros!
Overall I feel like being only in Cagliari alone for 6 days as a vegan GF person without a driver's license that doesn't speak Italian was not my best idea ever, but it had its cute moments and I hope other visitors will have a better time than me.