Formal:
* The following day
* The subsequent day
* The day after
* On the morrow (archaic)
* Twenty-four hours later
Informal:
* The day after that
* Tomorrow (if you're talking about the day after today)
* The next morning
* The next evening
* The following morning/evening
* The next day, after that
More descriptive:
* The following Monday
* The next morning, when the sun rose
* Later that week
* A few days later
* A week later
The best word to use depends on the context of your writing. If you're writing formally, you'll want to choose a more formal word or phrase. If you're writing informally, you can use a more casual word or phrase.