You can never have enough memory cards (I have 10 2GB & 4 1GB cards) and I also have 4 batteries (I have a battery grip that can hold 2 batteries, though I don't use it as much any more because it doesn't have a shutter release,,,, Nikon didn't make one for the D50 {or the D40, D40x nor the D60 I believe, but there are 3rd party ones).
For x-mas I finally got a "real" tripod. I had been using the inexpensive ones that are found just about everywhere, but it really wasn't very sturdy. I got a Manfroto tripod and ball head.
Get yourself a remote shutter release.
I have a Tarmac Adventure 9 backpack that holds all my equipment together in 1 place, though it is kind of big to use in the parks. I'm looking into getting a smaller bag for day trips where I wont be bringing all my stuff.
I have a few filters, but don't use them much. Its probably a good idea to get a circular polarizer. Thats the one I use the most. I also have a UV filter (rarely use) and 4 stop ND filter (occassional use). There are many other types, but that is up to the individual and their shooting style.
I also have 2 external hard drives (actually 3, but 1 I don't use for storage) to use as additional backup for all the image files. They build up pretty quickly especially when shooting RAW.
After the 1st trip to WDW with my D50 I got an Op/Tek neck strap. SOOOO much better than other straps and makes the strap that comes with the camera feel like nails around your neck. Its definately worth the $20 or so.
Make sure you have an online account to share the pictures with friends & famile (and the DISboard)

Some you can use as additional backup storage.
Did I mention making sure you have plenty of backup storage (external hard drives, online storage, CD's, DVD's).
As for which lenses will be used most. That depends on you and your shooting style. Some people like shooting at the long ends, other perfer the wide angle. On my previous trips I mostly shot at the wide end. Most of the long end shots were at Animal Kingdom.
Once your more familar with your camera and your shooting habits, other accessories will be more obvious than others for your style.