If you need carseats and have
AAA, rent the vehicles through the AAA website. You can get free car seats from Hertz and its 2 other partners.
There is a website I used (not sure if I can mention it here) that tracks car rental prices and will email you if it finds something lower. You can do a web search for "track car rental prices" and the website should be there. Beware of this since prices fluctuate hourly and you could get a lot of emails, even for a penny savings.
An alternative is to do the search daily on a consolidated website like Expedia and you'll see prices fluctuate (but book direct with the company). I've saved a lot using this method.
Last, change the times slightly for your searches to see if you get different rates. For example, if you need the car from 2pm to 10am the next morning, do a search for 2pm to 2pm and return the car early with no penalties. I've learned that prices could differ just based on the times that you enter.