My dad did a job for his neighbor a few years ago where he covered their garage with that "shower board" stuff. (Everyone calls it shower board because it has the same satin finish as some shower enclosures, but it's actually a hardboard product and should not be used in wet environments.) It's $12 per sheet (barely more than sheetrock) and you can buy strips to tie multiple together as well as strips to terminate a wall. It's quite attractive, VERY bright, and requires no finishing work. I helped him with the last stage of the project and it came out really nice.
If you have a budget for even better materials, a very cool option would be roll-formed steel wall panels. I used to work in architectural sheet metal and we had a machine like this one:
http://www.zimmerman-metals.com/roll-form/products/ws-1000-wall-soffit-panel-machine/
The final result is beautiful, and it's used for walls and ceilings. It's like a much more durable and rigid version of soffit. We used it for spans up to 150 feet (continuous, formed on-site). I've actually used some leftovers as a cool white board for a project at one point, and it's obviously also a great magnetic board. And it's very strong, so you can attach things to it with sheet metal screws, such as cabinets, shelves, lighting, etc. For a residential-appropriate lengths you could find a shop with one of the machines and have them run it at their shop and you can install it yourself (using steel-to-wood wafer screws from any fastener supplier) to save money. And there are many color options from many suppliers, such as Firestone and ABC:
http://firestonebpco.com/technical-information/colors/
https://www.abcsupply.com/BranchLocator?state=New Jersey
Just an idea, as long as you're open to suggestions.