72 hours in St Petersburg, can I do these things?

We visited by ferry in 2014 and did the following:

Day 1 - By the time we got through immigration, took the shuttle through lots of traffic, and checked into our hotel it was almost noon. We headed by metro to Nevsky Prospekt, had a quick lunch, and spent the afternoon walking along Nevsky Prospekt and visiting various cathedrals and sites. Had a great dinner at a Georgian restaurant near hotel.

Day 2 - Joined a small group tour. We were picked up at hotel. Tour included the hydrofoil, Peterhof Gardens, Catherine Palace, and Peter and Paul Cathedral.

Day 3 - spent 3 - 4 hours at Hermitage (got there before it opened), had lunch, and then headed back to hotel to catch shuttle to ferry

Peterhof Gardens is open late, so you might do that on your first day. The pier for the hydrofoil is pretty close to the Hermitage so you could visit the Hermitage for a couple hours and then take the hydrofoil to Peterhof. You will need to do Tsarkoe Selo on day 2 and I'd suggest you hire a guide or join a tour so that you can get out there efficiently and make the most of your time. On day 3, you can visit sights in the city like St Isaacs.

I did the ferry trip from Helsinki in 2016. FYI, it's a little different now - same visa rules, but the old ferry operator (St. Peter Line) was bought out, and the newer operator operates the ferry in a different schedule. When I did it in 2016, we got off the ferry in St. Petersburg in the morning as soon as we arrived, spent two nights, and got back on the ferry to return to Helsinki late in the afternoon of the third day. Now, I believe, you truly get three nights in St. Petersburg, but you must wait on the ferry docked in the morning for several hours before you can get off (unless you have a visa), and when you return 72 hours later, you again have to wait on the ferry a while until you depart in the evening. This gives you more time in St. Petersburg (an extra evening), but means a whole lot more waiting around too. But the ferry experience itself was great - I loved it, much more than expected.

I can't tell you how long it will take to do various things, because you and I have different interests. I am not a museum person at all. I still visited both the Hermitage and the Russian Museum, but I didn't spend more than a few hours in each (maybe an hour at the Russian Museum). I didn't visit Peterhof, either. Mostly, I just wandered around town and explored the city. I highly recommend wandering around at night to see St. Petersburg when all lit up - very beautiful, especially along the Neva and along the canals. (Unless you visit in June where it gets dark really late, if at all.) I was surprised how friendly Russians were to me when I was out and about - unfortunately, I don't speak Russian, because I could have had some fun conversations I think.

St. Petersburg is very touristy, more than I expected. (Lots of Asian tour groups.) Not many Americans, though, of course, due to the visa requirement. But it's easy and relaxed getting around - doesn't feel much different than exploring any other European city.

I recommend having a working smart phone in St. Petersburg. I used Google Maps to navigate around by foot and by bus. St. Petersburg buses are cheap but may be intimidating at first, but they are easy if you have Google Maps telling you which bus to get on and where to get off, etc. You don't need to read a Cyrillic bus schedule or map! You pay 30 Rubles in cash to the bus conductor (or the driver) when you get on, and they give you a paper receipt (no transfers). I found it every effective and simple and a time saver for sure. You can make a map ahead of time on Google Maps of the places you want to visit in St. Petersburg, your hotel, etc. then just call them up at random on your phone and find the best way to get to the next spot. Many people use the subways but I didn't find the need for them in the places I went.

Actually, you really can find time to visit all these places just in 72 hours, but you will have to strain. In St. Petersburg I recommend to visit the roof tours . There you can find an excellent view of the city. And after that you don’t want to rush anymore.
Of course, you can do it by yourself. We mean organize your roof tour without the special guide etc. But we used the services of alltheroofs.com and we don’t regret it.