A city tour in Porto Alegre is available, but you need to make reservation. Their phone nrs are (51) 3289.6744 / (51) 3289.6745. It´s made on an roof open bus and it´s narrated. Leaves at Secretaria Municipal de Turismo (Travessa do Carmo, 84 - Cidade Baixa) and costs about R$ 15,00/person. Takes about 90 min. If you speak Portuguese, try …rs.gov.br/turismo/default.php…
Regarding Iguaçu Falls, take a look at Google Maps and you will see that is far from Porto Alegre. Depending on how long you are going to stay in Brazil, maybe it is better to take a plane from Porto Alegre to Foz do Iguaçu. Take a look at http://www.cataratasdoiguacu.com.br/
Gramado is about 90 min (by car) far from Porto Alegre. You may try to know other cities around. If you speak Portuguese try …rs.gov.br/turismo/default.php… or contact Porto Brasil Travel Agency (http://www.portobrasil.com.br) and check about the tour on Serra Gaúcha. That includes Gramado.
It all depends on how long you are stayng, budget and time of the year!