Hi, Tracie —
Giving something up for Lent, with the exclusion of:
- meat on Fridays and Ash Wednesday, and
- fasting on Ash Wednesday and Good Friday
is a pious custom but is not found in canon law or the tradition of the Church. Hence there are no guidelines or regulations concerning it and no right answer to your question.
The rationale for the practice you mention — indulging on Sundays in whatever it you gave up for Lent — is that, supposedly, if you count the days in Lent, you get more than forty unless you subtract Sundays.
Ergo, Sundays are not, the argument goes, a part of Lent. That said, this has never been the Church's liturgical practice. Sundays are clearly a part of Lent and are named as such and use a penitential color and so forth. The bottom line is, since giving something up for Lent is purely voluntary, how you want to manage it in your personal life, is up to you. If you want to cheat you aren't less of a Catholic for doing so, nor are you violating any laws or traditions.
For more information, see