If you've purchased an iPad or iPad 2, you're going to want to go with Bluetooth since the Apple tablet lacks any USB ports. However, if you're planning on picking up the Kindle Fire, skip the Bluetooth since Amazon's tablet doesn't support this.
Whatever you choose, you can maximize your dollars by getting a keyboard-slash-case. These devices, such as the Kensington KeyFolio Pro, provide a degree of protection while enabling you to prop up the tablet while typing away. They also often have multiple viewing angles, which can be helpful while watching videos.
Many portable keyboards also pull double duty as stands, but there are other options out there as well. Consider investing in a stand or mount for your car's headrest so the tablet can be used as rear-seat entertainment for the kids. Many of these headrest stands come with inputs or IR technology for use with headphones, so you won't have to listen to [insert most favorite movie here] 10,000 times.
You're likely loading up your iPad or Nook Color with music and videos, so get the most out of your sound with a portable speaker. If you have the iPad, Bluetooth is again your best option, and you have a variety of choices, including products from Logitech and Creative. There are multiple USB-powered portable speakers you can get if don't have Bluetooth, and this site has reviewed a number of them: