It is a great deal. I bought a 24 hour card and used it for 3 days. They will also give you a pen or two. There is a public restroom on the corner between the downtown bus station and the Tourist Center. It is underground and very clean.
Salt is a spectacular souvenir and it is extremely tasty.
It doesn't take up much room, adds a few pounds to your bag, is legal and cheap, and a great gift. The Family Zoo has barnyard animals and reindeer and seals. Get the feeding schedule when you use your Reykjvavik bus pass and get in for free. See the Icelandic horses up close and throw the seals a fish. The Blue Lagoon is a must.