Crowds
First, IMO people should not be as scared of crowds at DLR as many of them are. Crazy busy crowds that exist during Christmas week and New Years, Thanksgiving, Veterans day weekend and many holiday weekends in general should be avoided if possible. On such days DL may reach capacity at something like 75000. But aside from these extremely crowded times, normal high season crowds are very manageable IMO with a good game plan. A more "normal" high season weekday at DL will have 40000-50000, and thus be substantially less busy than the days with 75000.
Number Of Days
I think three days is a minimum number of days and you will be happy if you choose four days at DLR.
Shows
Most shows are typically available all year on weekends as you and I have discussed on my other thread. This year is different with Fantasmic down because of the work on Tom Sawyer Island converting it to Pirate's Lair, and because Fantasmic is performed on TSI they had to take it down. Normally Fantasmic runs year round.
The Disney Electrical Parade is seasonal - I think they brought it back to DCA in March this year because of Fantasmic being down. Otherwise it varies year to year and is usually only performed during true high season.
They are working on a new blockbuster night time show at DCA called "Wonderful World of Color" and that could be available in summer 2008 according to rumors. That might make DEP go away "permanently", with qualifications on permanent because DEP has gone away permanently before only to come back after a few years.
Aladdin at DCA is year round, but FWIW it was down for 4-6 weeks last year in Sept-Oct to recast the show. So things like that can happen, and if they do it will be off-season.
In summary, if you avoid the high season and make at least one of your days be a weekend then there is a good chance you can see all the shows over your four days. If you go high season you will have the very best chance of being able to see all the shows but it will of course be more crowded.
Rides
Of course you can never guarantee all rides will be running when you are at DLR, but you can put yourself in position to have the maximum chance that the maximum number of rides will be running.
There are two time periods you can do this: Summer high season (late June to early Sept) and Holiday high season (Thanksgiving to New Years).
The last couple years DLR has taken rides down during Spring Break high season because some refurbs require four months and there is no way to fit that around Spring Break, Summer and Holiday high seasons. So Spring Break is the high season most likely to have rides down. In 2006 this meant POTC being down, and this year in meant Splash Mtn being down (late Jan until late May) and TSI and Fantasmic (a show) being down (late Feb to late May for TSI and to mid-June for Fantasmic).
There are already rumors about Indiana Jones going down in early 2008 for four months for an extensive refurb something along the lines of what happened with Splash Mtn this year.
Note that some people have visited DLR in a traditional off-season period over the last couple years only to find the parks much more crowded than they expected. Some people are saying that off-season no longer exists at DLR, which may be an exaggeration but has some truth in it.
Weather
If you want warmer weather for swimming then you may want to stick with April-Oct, with April and Oct being more risky for cooler weather.
Summary
You can't have it all! If you want maximum rides and shows and guaranteed swimming weather, that means a summer visit with higher crowds. If you are willing to risk a ride or show being down in order to get lower crowds, then a mid to late May visit may work best for you as long as you overlap a weekend to catch the weekend only shows. Typically with May being so close to summer high season that means many of the refurbs are done by then.
If I were planning a May trip it would probably be the weekend before Memorial Day, and if I planned four days it would likely be Wed-Sat. That gives two true weekdays on Wed/Thurs, and Friday which is usually not too crowded during the day in off-season but is at night, but also has the shows at night. Then Saturday would be crowded but also have all the shows.
Note that such a plan this year would overlap with the POTC 3 movie premiere at DL on Sat May 19, so you have to watch out for special things like this.
Sept is a lot like May, and typically has nice weather with lower crowds. But Haunted Mansion is always down for most of Sept to add its holiday overlay, and often Matterhorn is down as well. So between May and Sept I would choose May if I had your priorities.
Oct is also an option, but IASW goes down in early Oct for its holiday overlay, and there is a greater chance of cooler weather.
