Yes, use a high chair. Pretty much any decent one [1] will do since you aren't spending a full day in it (but get at least a decent one, not a cruddy one).

1: http://www.overstock.com/search?keywords=drafting+chair

I far prefer a high chair to an adjustable desk. Adjustable desks involve too much of a mental decision and then a couple minutes of downtime every time. And added cost.