Kungälv is pleasantest in summer (June–August), when the old town fills out and the riverside terraces open. The outdoor classic car evenings at Gamla Torget run from mid-May to early September.
The standout annual event is the Kongahälla Medeltidsdagar (Medieval Days), held the last weekend of July. The festival takes over the area around Bohus Fästning with a medieval market, craft stalls, a jousting tournament, period food, and a public wedding ceremony on the Friday. Participants are encouraged to dress in medieval costume, though no one will turn you away for wearing jeans. Cash only — no credit cards accepted. Details at medeltidsdagarna.se.