When traveling it is best to utilize the time as best as possible. This can mean planning in advance, making a schedule, and sometimes renting a car. Obviously, if the vacation is centered around a big city, then a car will only slow you down, but here are the circumstances when it is best to be mobile and independent.
If public transportation is not very good or slow, then that would be a good time to rent a car. Smaller cities where transport is infrequent or ends early in the day are good examples. Most vacations have limited time and there is nothing more frustrating than waiting 2 hours for a bus, or having to cancel plans because of bad transportation!
Another case in which a car is very useful is if your travel plans involve lots of nature and mobility. A trip with multiple hikes or visiting different ski resorts involves a lot of mobility, and an hour or two delay in the morning could possibly ruin a full day of outdoor activities.
Lastly, when traveling with multiple friends, many times it comes out cheaper to split a car rather than having 3 or 4 people paying for public transportation. Local car companies are usually cheaper, but Kayak and rentalcars.com are two easy websites to use for general prices in almost any city. It can be cheaper and easier, and a rental car is worth a quick check before the start of a trip!
