I was in Walmart today and saw a great deal that'll be loads of fun and might be exceptionally useful to someone with more skill in electronics than I. A 1/9th scale Abrahms M1A2 tank for $80. It was originally $200. The battery in it (a 12V 7Ah) is worth around $40 so it would be worth buying just to retrieve the lead/acid combo. This is a relatively lifelike model from 21st Century Toys. I couldn't find any info on their website about it so I assume it was built exclusively for Walmart. It is amazingly huge: 3' long with a turret that measures 12" by 16". The battery takes 18h to charge, so we haven't run it yet. It'll be interesting to see how well it runs and for how long.
I haven't taken it apart yet but it looks like it will easily fit a laptop and GPS. I'd bet with some skill it would be a fantastic, cheap robotics platform. If the boys let me, I'll take it apart and post some more pictures and specs on its mechanics.