Try going on KAYAK. (flights, hotels and transpo)
It will search most all of the "travel engines" simultaneously.
Buy your rental car contract/agreement online.
Let's say you chose an econobox vehicle to be thrifty.
If the rental car company is out of that class of vehicle, they might not tell you at time of pickup.
Instead, they may try to upgrade you at your cost.
Beware of that.
If they don't have your vehicle class in stock they have to upgrade you at their cost.
This has always been my experience and I've had the "upgrade trick" played as well.

You typically don't get a very good deal if you walk up to the counter cold-turkey at your destination.
Figure on a butt rape.
Better to hear the clerk say, "Wow, you got a good deal !"

I usually end up with Hotwire or Travelocity. (or a combination)
I try to look them all over, though.

If you want to fly Southwest, you have to go straight to Southwest.
They don't play the travel search engine game.