The Cybiko Classic is an excellent handheld device. It features an LCD screen, a full qwerty keyboard and an RS232 port. Great for use as a handheld terminal - just load the fine (and free) terminal emulation software from www dbzoo dot com and you will be up & running.
Of course, with a full SDK available, you might want to code your own stuff to run on the Cybiko.
Be aware that many units selling on eBay suffer from dud batteries. Either buy new ones, of see other guides for details as to how the batteries can be 'refreshed'.
Note that the Cybiko Xtreme has no RS232 port - the 'Xtreme' unit looks rather more modern that the 'Classic' one, but if you are looking for serial connectivity, the Classic is the easiest way to go.