I have hired a car a number of times and have used Enterprise, Hertz and Alamo/National. I have only had a problem once and that was with Alamo/National (after having no problems twice before with them).They would not allow me to use a debit card for the hold (you don't get charged until you return the car) and stated that it must be a credit card (even though I can run my card as either debit or credit). I then left and tried out some of the other rental companies and they all told me the same thing. Apparently at the airports a credit card is required but if you go to any of their other locations it will be fine to use a debit card. So I would suggest calling up these companies to find out before you get there and are left in the same situation that I was in.
Also, renting from their 'in town' locations, i.e not at the airport, is going to be cheaper (I have found they generally have a higher return fee on cars being returned at the airports).
If you are driving the car one-way, you will get charged a different location drop off fee.
As for hidden fees and what not: If you are under 25 you will be charged an underage fee, which is somewhere around $10-$15 a day (!!!!!). Simply ask them to waive this for you and they should. I have rented a car 5 times in the past 6 months (I'm doing a big trip across the USA) and have never been charged the underage fee. If there are two of you who want to drive the car, then you will be charged an additional driver fee. You will also have the option to pre-purchase fuel, that way if you forget to fill the car back up before returning than you wont have to pay their excessive prices of like $6 per gallon, but that's up to you.
As far as insurance goes, you will have a few options for this. I always go with the basic cover, so if the car is damaged it is covered but if I hit anyone than they other car is not covered (and I would have to pay for the other car's repair costs). So I just have to be very careful. Also, as a driver/passenger I am not covered under the basic coverage (if I get injured, etc) but I have comprehensive medical and travel insurance that would cover that. I'm not 100% on how much the insurance is, but it's somewhere around $20 a day I do believe.
From memory, the last time I hired a car for just a day it was just under $100 I think (that was DC to North Carolina). And hiring for a month at a time has been around $1800.
I hope this helped.