In this week's parsha Hashem commands ושמרו בני ישראל את השבת... לדורותם ברית עולם (לא:טז), that Bnei Yisrael should keep and (לעשות) do the Shabbat. However, how does one "do" the Shabbat? Shabbat is not an activity that one can physically perform.
Responding to this question, the אבן עזרא explains that one should take care of any preparations that need to be done for Shabbat. Therefore, any of the cleaning or cooking that must be done for Shabbat is the לעשות to which the Torah is referring.
Another answer is suggested by the מלאכת מחשבת על התורה, a philosophical work of the early 81th century. He asserts that when one has an obligation to do something which requires no action but exclusively thought, completion of the required thought is referred to as an "עשייה", an action. The main reason Hashem gave us the Shabbat, explains the מלאכת מחשבת, is to remember the creation of the world, that Hashem created the world in six days and rested on the seventh. Thus the Torah states in the next pasuk (לג:יז) ביני ובין בני ישראל אות היא לעולם כי ששת ימים עשה את השמים ואת הארץ וביום השביעי שבת וינפש, that Shabbat should be a sign for all generations that Hashem created the world in six days and rested on the seventh. Hence, concludes the מלאכת מחשבת, the commandment of לעשות את השבת is one that can properly be fulfilled through refraining from melacha and demonstrating recognition and belief in this idea. Therefore, when one has fully attained this belief, it is considered as if the person is physically and actively fulfilling the injunction לעשות את השבת.