Hey I figured I would offer some help as I currently work for National and Alamo and have worked for
Enterprise in the past.
As far as the Contract ID's from other places (IE Costco, Ohio State etc). 9/10 times no one will ask you for an ID or proof but like someone said earlier, some branches will ask for an ID to show them. It really depends on the employee or the branch because they are supposed to do that every time but some airports deal with thousands of reservations in a single day and don't ask because they want to move through the reservation to get to the next person. Also I don't promote anyone using a discount for an employer they don't work for but if they do and something happens to that car, they might call that company and verify employment and if it's business use or not. I know my region does.
Try a discount with your employer or even their parent company. For example, you work for Duracell and there isn't a discount code but you try their parent company, P&G and you might have luck finding something. Even if it's a business discount, most of them will allow you to use it for personal use as well, they just won't cover damages on personal rentals. I can also look up if there is a discount for your company or organization with National or Alamo, just PM me.
Costco and BJ's are also good because they offer coupons for upgrades, free additional drivers, or $$ off.
Also most companies have loyalty programs that are free to sign up for and offer 5-10% discounts right away as well as loyalty points towards free rentals etc. Emerald Club is my personal favorite but that's me.
If you own a small business, call a local branch, there are discount codes for that too from 5-10%. Just ask how to sign up for the small business discount and it will generate a code for you to use.
There are generic local 5, 10, 15, 25% codes, I'll see if I can find them again
In the off season, Enterprise branches offer weekends specials for as low as $10/day. Yes they are usually only sedans and have a mileage limitation, but if you're staying in Orlando, you probably won't exceed that. Keep in mind that's usually not at airports and only at some offsite locations. Which might not be a bad thing to consider because of the lower taxes and fees etc.