Make your passcode more complicated: iOS defaults to a four-number passcode, but you can turn it off by going to Settings, General, Passcode Lock, Simple Passcode. Now you can use any full keyboard password to lock the iPad. The password also helps to encrypt your mail and attachments on the iPad, so you probably want to make it safer than a four-digit number.

Other interesting tips:
Choose an app for e-mail attachments: You can open a file attached to an e-mail message by tapping the attachment in Mail, but if the default app isn't the one you want to use, you can press and hold and wait for a menu that lets you select an app.
Forget Wi-Fi networks: So you accidentally connected to a network once, and your iPad remembers it for life--whether you like it or not. On your iPad, go to Settings, Wi-Fi, and find the network under the 'Choose a Network' heading. Tap the blue arrow next to the network you want to remove, and tap the button on the top that says Forget this Network.
Specify your Spotlight searches: The iPad uses Spotlight for its built-in search functions, and you can tweak it to your needs in Settings, General, Spotlight Search. If you have a lot of data on your iPad, for example, you can selectively disable search in different categories (Contacts, Applications, Audiobooks, Notes, Events, Mail, and so on) that you don't use so that your desired results show up faster. You can also simply change the order in which the search-result categories display by dragging them up and down, so that your more frequently used search categories show up at the top of the page.
Change your e-mail signature: Don't be one of those people who leave the default 'Sent from my iPad' signature on all their messages. Change it in Settings, Mail, Contacts, Calendars, Signature to...well, anything but that.
Take a screenshot of whatever is on the screen: Press Home and the sleep/wake button simultaneously. The screenshot will automatically appear among your photos.