My advice is if you can afford it, stay in the hotels in the theme parks. It's nice to abandon the car and not worry about transportation. Disney has an awesome transit system for all their theme park hotels with the monorail going through many of them. Universal Studios has a boat that picks everyone up. But you'll get a lot more advantages too including a quick line pass, which will get you in to many of the theme park attractions quicker than other guests. Disney even allows people who stay at their hotels to enter the park an hour earlier and stay at the parks an hour later than off-park hotel guests. If you don't stay in a hotel in the theme parks, pick a hotel nearby the theme parks that has 15 minute or quick shuttle departures. Find one that goes to both of those theme parks, Seaworld, and other places and you'll have a much more relaxing vacation with no driving required.
If you have little kids, Disney is a must. Universal isn't as little kid friendly, especially their Islands of Adventure theme park which has many roller coaster type rides that require a certain height. Don't buy anything more than a 2 day pass to Universal, and if you go during the offseason (like right now!) and have a good map, you can actually ride everything at Universal in one day if you go from morning to dark. Disney, I'd recommend a 3 day pass minimum and concentrate mainly on Magic Kingdom, Epcot, MGM and Animal Kingdom. Seaworld can be covered in 1 day.
The nighttime entertainment stuff is okay, but nothing special. If you're into that, Vegas, New Orleans and other places are much better spots but if you have energy and time, check out Disney's Boardwalk, Downtown Disney and the similar attraction at Universal. Most of the restaurants are overrated. Mostly stuff like Planet Hollywood, NASCAR Cafe, Jimmy Buffet's, and other celebrity chains that are glorified overpriced Bennigans that you really only go to if you're into the theme of the place. If you really want nighttime entertainment, I'd recommend downtown Orlando over the theme park stuff.
Lots of Vegas style buffets all around the theme parks if you'd like a large helping of lobster, steak and sushi. Lots of dinner shows everywhere. Everything from Dolly Parton's huge western extravaganza, to Arabian Nights Sinbad-style fights, to those medieval themed places, to mystery dinner theaters. Several mini theme parks are in the city. Wax Museums, big go-cart races, helicopter rides, small theme parks, and even a Jesus Park where the people and buildings are decorated to look like events in the Bible.
For shopping, check out Millenium Mall which is a mall that has Macys, Nieman Marcus and every exotic or high-priced type of shop you can think of. It's something you'd only find in a huge city, not Orlando, and only there because of the tourism. International Drive (near Universal) has a lot of nifty shops and restaurants. Downtown Disney sells everything Disney related you could ever think of. Check out The World of Disney shop if you need to pick up stuff for friends and family.
Gas, fast food, and all the simple things are sometimes doubled in price near the theme parks so if you're looking to save money, do those things away from the entertainment areas. Keep some quarters handy if you're driving into Orlando, there are tollways into the city. May will be busy, but not nearly as busy as June-August. Spring break is busy. Christmas is busy. Halloween is busy. Especially Universal Studios which turns itself into a HUGE haunted house park. Slowest time is around now and immediately after summer.
If you really love roller coasters, check out neighboring Tampa Bay and the Busch Gardens theme park which is basically a huge roller coaster park. Tampa has a lot of fun stuff to do too. Check out Cape Canaveral which is about hour drive away from Orlando. The beach is an hour away too. Key West in south Florida is an awesome place to snorkel, dive and nightlife away. Hope the advice helps.