Purchase the Göteborg Pass. (formerly the 'Göteborgskortet') Other than being a cheap yet cool souvenir, it really comes in quite handy. For SEK 175 ($20.00 USD) /day-Adults, (SEK 295-2 days) or SEK 95 ($10.50 USD) /day-Children, (SEK 190-2 days) you're granted unlimited local transit, free admission to Liseberg and various other attractions and Museums, as well as other little advantages which make it a little easier to travel around for a day. Besides, who would'nt want a little plastic card with a silhouette of Göteborg's founder; Gustav II Adolf, to carry around in their wallet?
-Secret tip: Now, I'm not one to advise ripping off anybody, but if you 'forget' to fill the card out until someone asks you to, then you just might be able to use it for more than the 1 day. You did'nt hear this from me though; I'd never think of such a thing. ;o)
Official City Site