I'm a great fan of public transport when it's done right. Case in point, Portland's MAX light rail system (read: tram). We decided on a short weekend away and rather than renting a car, we decided to rely on public transport instead. There's a MAX station right at Portland airport, and the red line goes into downtown Portland, within a block of most of the hotels. We hopped on, paid $4.60, and hopped off 40 minutes later within two minutes walk of the hotel. On the way, we watched gridlocked rush hour traffic, nose-to-tail on I84 and when we got to the hotel, we discovered overnight parking was $29. In this case, it's a total no-brainer. Sit in traffic, pay three day's car rental ($150), two nights valet ($58) and half a tank of petrol ($15), or spend a grand total of $9.20 on public transport to do the exact same thing.
Think about it next time you go away somewhere - do you really need that rental car?