We anchored overnight in Porto Palma (not a port, only a bay), in the S of Caprera Island, w/o a
permit, in late April. There was another sailboat there, at
anchor. Isolated spot, not much to do. Scenic.
Also anchored overnight at Cala Ferrigno, Isola Spargi, in late August. We had a permit which expired at midnight, so we were subject to about an €80 fine, if I
recall correctly, after zero hours. As we departed at around 8:00, we had a free stay. I do not
recall that anchoring was prohibited, but maybe it was, and if so, our exposure to fines started at sunset, and not at midnight. Nice shore walk to a WW II pillbox and to a small bay.
Stayed all day and overnight on a buoy in Passo Cecca di Morto, in front of the Isola Budelli
dinghy landing, in late August. Key to finding a buoy: arrive no later than mid-morning. After that, it is unlikely you'll get one. The revenuers came by not once, but three or four times during the day. They somehow remembered our
boat, because they only stopped at our the first time. I have read of others whom the revenuers somehow missed completely despite going multiple times to the neighbors'. Clearly a "must" location for any cruiser: stunning, calm (after the day trippers leave), beautiful. Walk on Budelli,
kayak or SUP around Razzoli or Santa Maria. Highly recommended.
I think some people make the mistake of only spending time in the Parco Nazionale. Guess what, the anchorages in the actual Sardinia "mainland", all the way from Porto Rafael to Salina (i.e., immediately N of Olbia), are all awesome and often beyond awesome and free. Cala Liscia Ruia, to mention just one example, just S of Cala di Volpe, is a stunning free anchorage that has nothing to envy the Parco Nazionale which stands across the
water 1.5 miles away.