--Elbtunnel will be open for walking and no cars throughout the festival. Lots of people use this event to walk through.
--Harbor cruise--may or may not happen during the weekend; the harbor is a zoo and the departure may be affected. You might also want to check out the Maritime Circle harbor tour--it is in German and English, and I have enjoyed this tour on a few occasions.
--Rathaus tour--not sure how the festival will affect it, but the Rathaus is a few blocks from the harbour, so it might not be too be bad. Probably not available on Sundays.
--MWunderland--always recommended!
--St. Nikolai--easy to access at any time. The museum in the basement about the fire bombing is interesting.
--Elphi will be super crowded during the festival. Wait until afterwards.
--Alster will be much quieter, so a good break from the festival.
Here is the thread--I am a few posts down. https://community.ricksteves.com/travel-forum/germany/hamburg-worth-4-days-with-day-trip-to-lubeck-or-bremen
That's just a bit. You can also visit places like the technically-a-suburb Wedel, Schloss Ahrensburg, Kiekeberg, Bergedorf, Ohlsdorf cemetery, the Botanical gardens in Klein Flottbek, or the orchards and half-timbered homes around Estebrügge / Jork. These are more local places to visit rather than typically tourist sites, but if things get too crazy they are options.
As for the S1, the exit is on the South side (if you are on the platforms between the two escalators up, go to the side with the giant PHILIPS sign, not the side that says Wandelhalle / McDonald's) and take the exit to the east (left). The S1 is immediately past the exit back down the escalator near the police station and the flower shop. It's marked clearly and is the green line. Your hotel is on the North side (Wandelhalle / McDs) but also on the east side. There is a tourist office near that exit, too.