I'm thinking of a laptop-like device with a pluggable ROM socket. And you could buy various ROMs for it, which would specialize in various things, and BAM, instant computer ready to go, immune to viruses, perfectly configured, and you could save things to a flash drive or something. Deep banks of RAM would ensure that one would never run low, and a solar panel would ensure that it could run for as long as you're in a bright room, which you probably are. If we used a power-efficient CPU, we'd never need to plug it in, ever. For safety, ROMs would be locked in place while the computer was on, and could be ejected after shutting it down. In the absence of a ROM, the computer would have a built-in ROM with basic computing environment.
Being a computer-science major, I can also speculate on the layout of the ROMs. Each would contain a XIP kernel with built-in filesystem, so extra RAM would not be needed to run anything on it. And each ROM would specialize in one task. There would be a writer's ROM, that had word processing programs, spell checking (in every language currently known), maybe idea generators. There would a be a sound-editor's ROM, with all kinds of audio-editing goodness, MIDI, real-time capability with JACK and the like.
There would be a science ROM with chemical analysis tools and biology computing tools and everything a lab could want a computer to do.
There would be an art ROM with tons and tons of 3d rendering and graphical editing tools.
There would be a programmer's ROM with development tools and hundreds of compilers and analyzers.
Almost any subject available would have a ROM that could be popped in and quickly run. It would be faster than a traditional computer, and need less maintenance. Although, on the downside, the software probably wouldn't be upgradeable. When new versions of the software are written, you'd have to replace the ROM.
There are three kinds of ROMs that could be used for this. Mask ROM is the cheapest, in which the chip is etched at the factory. This also means that once made, it can't be changed. Ever. The second kind are EPROMS, (Erasable Programmable Read Only Memory) in which the ROM can be blanked out by shining ultraviolet light on them and then re-written at the factory. This allows old ROMs to be recycled when returned, though it is beyond the customer's ability to do so. For the most money, EEPROMS are like EPROMS, but can be blanked with higher voltage, which can be done in the customer's computer. If the customer can do it, this eliminates the need to bring obsoleted ROMs back to the factory. (Though it may raise the risk of a poorly done upgrade that leaves the ROM useless, requiring a trip to the factory.)