Suddenly, you're credit card is maxed out.
Cheap Car Rental
Here's a pro-tip. Most universities have negotiated discounted car rental rates for their staff, students and/or alumni - and they post the discount codes on their website. Contact your university to check what codes they have, and if you can use them. They may even publish them on their website, for example, you can use University of Sydney's Thrifty discount codes and ANU codes right now!
Best transport to & from the airport
Car rental is often cheaper than alternative transport to and from the airport. In Melbourne, using the discount codes above, you can lock in a hatchback for ~$60 for a weekend. That's roughly the price of a one-way cab ride from Tullamarine or 2 return tickets on the shuttle bus. Basically - you'll save money by renting a car.
Getting around town
Using your rental car to get around is great, but navigating the CBD can turn into a massive parking fees. Don't worry - there's more savings to be had! You should look out for free transport systems for tourists, like the 555 in Sydney (running from Central to Circular Quay) and the Melbourne Visitor Shuttle (that loops around the CBD).
Free walking tours
Want more free options? I've got one more tip up my sleeve!
If you do want to take in a bit of culture but can't fork out the cash, most major cities offer free walking tours. Tours usually run each day from a central point in the city and information can be accessed from most hostels. In Europe, SANDEMANs dominates the free walking tour scene - check out the website for more info.