The Mishna (Moed Katan 11a) teaches us that ma'aseh uman (the work of a craftsman) mustn’t be done on behalf of an individual on Chol Hamoed. Only ma'aseh hedyot (the work of an ordinary person) that is necessary for that Chag is allowed.
Rambam (Yom Tov 7:14) writes that personal letters are considered ma'aseh hedyot and so may be written. While others disagree, the Rema (OC 545:5) paskens that one may write, though should do so in an unusual manner. The Magen Avraham suggests writing the first line on a slant (See Mishna Berura 545:35).
Whether printing from a computer is considered ma'aseh hedyot or ma'aseh uman is the source of much debate. The dilemma is that while certainly nowadays typing requires less skill than writing (see Chol Hamoed Kehilchaso 6:89), nonetheless printed works can look more professional than written ones. Is it the act or results that make the difference?
R’ Moshe Sternbuch (Moadim Uzmanim 4:301) considers printing to be ma'aseh uman and therefore assur.
R’ Moshe Feinstein (Hilchos Chol Hamoed Zichron Shlomo p78) held that typing cannot be considered ma'aseh uman and providing it was of relevance to Yom Tov, would be permissible. R’ Shlomo Zalman Auerbach (Shemiras Shabbos Kehilchasa 66:n209) compares it to using stamps which the Eshel Avraham allows. While storing data to a disc is improving the disc, and therefore tantamount to boneh, providing the work was necessary for Yom Tov (or would cause a significant loss, etc.), it would be muttar. In a letter to R’ Avraham Avraham (Nishmas Avraham OC 4:340:4) R’ Shlomo Zalman explained that displaying letters isn’t problematic, as it merely shows light. R’ Ovadia Yosef (Yabia Omer OC 8:48) paskens leniently, too for the same reason.
While the Acharonim don’t discuss playing computer games, it seems that it should be muttar as, like driving to an outing, it can be considered relevant to the Chag and no Issur is involved.