Sometimes I consume quite much drawing paper by doing calculations, sketching things etc. That’s why I bought an LCD based writing tablet (the Boogie Board) which seems pretty handy! From here you can watch a YouTube review from the same product I bought.
With this thing it is nice to lay down on a bed or sofa, read some interesting literature and sketch thoughts and calculations down. No need for rustling papers and external erasers. No even to change batteries (ever), since the erasing function consumes so little electricity from the internal battery (at least the manufacturer says so). Though back lights would be good since it is hard to see writings and drawings with poor lights. One should also be careful and avoid writing important things down, since this equipment cannot save writings, transfer them to other media etc. It also lacks a pen holder, but maybe it is easy to do it yourself.