The fill handle doesn't only copy formulas. Drag it from a value that's part of a pattern and Excel continues the pattern:
- Month and day names: Jan, Feb, Mar... or Monday, Tuesday...
- Text ending in a number: Week 1, Week 2... or INV-1001, INV-1002...
- A date: one date fills the following days.
- Numbers: one number is copied as it is. To count up, give Excel two numbers first (1 and 2, or 5 and 10), select both, then drag: Excel repeats the step.
The same works dragging right across a row. If a fill copied when you wanted a series, undo and start from two cells.