Formal:
* Vous me faites la journée. (This is the most literal translation and is very formal.)
* Vous me rendez la journée agréable. (This translates to "You make my day enjoyable" and is still formal.)
Informal:
* Tu me fais la journée. (This is the informal equivalent of the first formal option.)
* Tu me rends la journée plus belle. (This translates to "You make my day more beautiful" and is a more romantic option.)
* Tu me fais plaisir. (This translates to "You make me happy" and is a very common way to say it in French.)
Other Options:
* Tu es génial(e)! (This means "You're awesome!" and expresses joy and appreciation.)
* Tu es tellement gentil(le)! (This means "You're so nice!" and is a way to express gratitude.)
The best option for you will depend on your relationship with the person and the situation.