I liked Gothenburg a lot (I spent 4 nights there during summer 2022; see sight list at end of this post), but Stockholm is in a different league, I think, so I'd spend the extra time in the capital, possibly with a side-trip to Helsinki or Tallinn. Won't you have some spare time during the conference to do some local sightseeing in Gothenburg?
Rick lists his top sightseeing targets for a lot of major cities right on this website:
Stockholm sightseeing - click on At a Glance
Helskinki sightseeing - click on At a Glance
Tallinn sightseeing - click on At a Glance
You should buy ferry tickets from the ferry company(ies), but you can find all the schedules on the DirectFerries website.
The major ferry companies in the area are Viking and Tallink Silja. You'll probably want a cabin for any overnight trip. I've done two overnights between Stockholm and Turku, Finland, and one daytime trip between Helsinki and Tallinn. The cabins are small but comfortable, with their own toilet and shower. Fares are like airline fares--variable with demand. The cost is likely to escalate as the travel date approaches. It may double or nearly triple if you wait to buy you ticket until a week or two before you travel.
The ferry companies use different docks. In Stockholm they're in different parts of the city. Viking was more convenient, overall, for me, but there's nothing wrong with the Tallink ships. Public transportation will get you to any of the docks you need to reach.
Helsinki and Tallinn are both very worthwhile side trips; I spent over 10 days in each. Unless you are nutso about Art Nouveau architecture (prevalent in Helsinki), I'd prioritize Tallinn, which retains a lot of old architecture and is more distinctive.
Gothenburg Sightseeing
Kungsportsavnyn (Avenyn): heart of the city, with outdoor cafes, boutiques, restaurants.
Roda Sten Konsthall: Contemporary-art special exhibitions in old boiler house.
Kuggen, Lindholmsplatsen 1: Colorful geometrically tiled building
Linne: Gentrified area with cafes, antique shops and craft shops. Between Jarntorget and Slottskogen, including side streets like Prinsgatan.
Haga: Oldest suburb (17C). Beautifully restored 17C wooden houses on winding cobblestone streets. Cafes, antique shops and craft shops.
Vasastan: District with intricately decorated late-19C apartment buildings (some National Romantic and Art Nouveau) and appealing cafes. See Vasagatan and Engelbrektsgatan (especially low number near W end).
Kronhuset: 17C grand brick building now living craft center used for concerts and exhibits. Underwhelming in 2022 but may have developed since then.
Stadsmuseum: History museum.
Centralstation: 1856; oldest train station in country. Grand interior.
Tradgardsforeningen: Park with manicured gardens and paths. Magnificent Rosarium and Palm House.
Stora Nygatan: Nice walk with neoclassical buidings to W and park (across canal) to E.
Paddan sightseeing boats: 50-min Stromma tour from Kungsportsplatsen bridge to inner harbor.
Rohsska Museet: Excellent museum of design and applied arts. Asian influence prominent.
Carl Milles fountain, Gotaplatsen.
Konsthallen: Exhibitions of contemporary Art. Next to Goteborgs Konstmuseum.
Konstmuseum, on hill above city: Nordic art and Hasselblad Center for photography. Sweden’s best collection of 19C-20C French art.
Varldskulturmuseet, Sodra Vagen 54: Interesting museum of world cultures with temporary exhibitions.
Botaniska Tradgarden, Carl Skottsbergs Gata 22A, Slottskogen: Sweden’s largest botanical garden.