Not so much an onsen, more of a traditional indoor swimming pool under an impressive double domed roof. A 25-m basin, waterslides, Jacuzzi and a gym.
稚内公園
It is a large park spreading over the grassy hills west of the town, offering some commanding views over the area. The rope-way has been closed, but it is only a short walk to the top beginning at the Hokumon Shrine (北門神社). There are some nice walking trails to explore, and over 4,000 wild cherry trees grow in the park if you are here during Hanami (mid-May). To get an even better view you can ascend the 70-m-high Motoe Hiraku Centennial Memorial Tower (開基百年記念塔・北方記念館) 09:00-17:00 (Jul-Sep until 21:00) for a 360° panoramic view of the cape, the expansive Sarobetsu Plain, the Rishiri Volcano and even sometimes to Sakhalin in Russia. Below is the two-story Northern Memorial Hall, which has a collection of documents relating to the local area and Sakhalin. In the weird, small Ice House (ゲストハウス氷雪) you can take a footbath in ice, if that tickles your fancy.
宗谷岬
This is the northernmost point of Hokkaido, and aside from a small inaccessible and uninhabited island it is also the northernmost point of Japan. Naturally the Japanese have gone out of their way to commemorate this fact with the largely uninspiring Northernmost Point Monument (日本最北端の地の碑), a gray concrete triangle engraved with accomplishments. Right next to it is a Statue of Mamiya Rinzō (間宮林蔵の銅像), a Japanese explorer from the Edo period famous for his explorations and mapping of Karafuto (Sakhalin), and discovering it was an island. Nearby you also find an Old Naval Watchtower (旧海軍望楼) dating back to the conflicts over control of Sakhalin during the 1904 Russo-Japanese War.
港北防波堤ドーム
Built in the 1930s, this odd-looking breakwater is a 427-m (1,401-ft) arched hallway supported by 70 Roman-style pillars. It rises over 13 m up in the air to provide some protection for the strong winds in the area. It looks entirely out of place, but makes for some good photographs. In the summer months (Jul-Aug) it's sometimes converted to a lively market with BBQ and karaoke.
抜海港 ゴマフアザラシの越冬
About 300 seals have decided that the concrete boulders of this small fishing port was a good home. Surprisingly, they seem to get along fine with the fishermen. The city administration has put up a small rudimentary observation hut with windows and a video feed, so the many guests now pouring in don't disturb the seals. Self-service tea is provided free of charge, nice touch.
稚内市立ノシャップ寒流水族館
Next to the Science Center. Fittingly for Japan's northernmost aquarium, it's centered around 2 large outdoor tanks with seals and penguins. The indoor area is rather smallish with the two main tanks containing species from the oceans south and north ecosystems respectively - the balloon fish is kawaii! Feeding shows twice per day in the outdoor pools.
稚内市青少年科学館
Inadvertently cute museum, it opened in 1974 - and it shows - since many of the exhibits are really retro. The science section set up to teach kids about the "latest" progress in science you can skip, it's out of date, and any kids won't understand the Japanese anyway. More interesting is the Arctic section with details of Japanese exploration of Sakhalin and the Arctic regions. A whole hall is dedicated to the famous (in Japan) Taro and Jiro (See the Wakkanai park entry). Also of some interest is the Planetarium with shows 2-3 times per day.