First, go into iTunes Preferences, Advanced, and tell it to not automaticaly sync your devices. This will prevent accidental syncing before you are ready.
With the iPad connected to iTunes go to the File menu and choose Transfer Purchases From...
This should move all your music, apps, movies, etc to iTunes.
Now go through all the sync settings, info, photos, etc, and turn them off. This will prevent you from losing stuff that you are going to have to move the hard way.
If you don't have too many photos, you can mail them to yourself, 5 at a time from the Photos app.
Another way to get back you content is to use
iPhone Explorer it will take a bit of digging around, but all your pictures, music, Video's, etc are stored in the Media folder. Copy the Photos and DCIM folders back to your computer. If you have an photo program that will allow you to import, then you might want to use it on these folders. Otherwise you'll have to dig through the subfolders and move the pictures to where you want manually.
You will find the Music and Books stored there as well. I recommend using the Transfer Purchase option mentioned before for the music and Books. If it works it will keep everything nicely organized when placing it back into iTunes. If for some reason it does not work, then you can get your music back the same way as the Photos. Copy it to the computer. You could then try the Import feature in iTunes to add them back to your library. I haven't' done this, so I don't know how well it will work. You'll lose your ratings, playlists, album art, and a few other things doing it this way.
Books purchases in iBooks should work with the Transfer Purchase option to. Same story as Music if they doesn't, except I have even less experience there so I have no idea how much trouble it would be to get them back into the library.
Good Luck
Note: I notice that under the Photos folders the pictures are stored under the subfolder Thumbs. These appear to be the full sized photos despite the folder name. However, they are probably not the same size as the originals you had/have on the computer. Photos are optimized for iPad display when synced through iTunes. any pictures you downloaded, stored, took snapshots, saved from art programs, are all in the DCIM folder, not the the photos folder.
