I'm familiar with the Gertboard, but honestly I had no idea there were so many accessories for the Raspberry Pi. You have to admit, that's pretty darn cool. It just goes to show that if you put something like Raspberry Pi out there, engineers will innovate and build on it.
This really reminds me of the days of the Apple II, Kim, IBM PC, and others. People were doing interesting things with computers that provided them the control that they wanted over their tech. The only difference is that the last generation is not saying silly things about how these computers are too small to be useful.
I see this generation creating a new set of technology building blocks that are going to challenge the Microsofts, Apples, and Googles to keep up. You don't like what they are offering? Build your own version the way that you want it. 3D print the mechanicals and assemble your own control systems for it. Computers used to be mysterious boxes, but now they are familiar technology. Embedded systems used to be mysterious as well, but the free tools now are better than what we paid a lot for not long ago and the technology is much more accessible. I've got to wonder what the next wave will be like...
the easiest option is to use a rear view mirror screen which takes UHF video from the PI. You can get these through eBay for $30 or so and take very little work to connect to the Pi (and no software). You can then tweak the Pi video settings to perfect the alignment to the screen. This gets you VGA level graphics. The only pain is these are designed for 12V (to be run from a car's power system) but some variants actually are 5v inside and with some bravery you may be able to steal that from the Pi connectors too.
IT would be an entertaining activity to build a stand alone data acquisition system. Use the tank as a motor, Battery, attach a robotic arm, and have the Gertboard ready to measure stuff. It could walk around and pick up soil samples, test them, send the data wirelessly, and move on...
The article misses one of the most capable io expansion card. http://roboteq.com/i-o-expansion-cards/rio-smart-io-expansion-card-for-raspberry-pi
Demo video at http://youtu.be/cFqNjndLtqg
And the kickstarter project that made it happen http://www.kickstarter.com/projects/95547492/smart-io-expansion-card-for-raspberry-pi
Here's another useful RasPi board I've come across, though I haven't tried it myself. Guzunty Pi is an "I/O Extender" board using a Xilinx XC9536XL or XC9572XL CPLD. It's actually sold as a kit, with a 44-pin PLCC socket for the CPLD and through-hole components for easy assembly. You can download various CPLD images for common I/O configurations, or program it yourself using free-as-in-beer Xilinx tools to perform any function you want (that fits in the CPLD). Board is GBP 2.60 plus shipping or GBP 10 plus shipping for the whole kit.