Things you can do:
- Sing louder / sing softer.
- Sing ahead of the beat / sing behind the beat. (rubato)
- Change the tempo.
- Increase the sense that the notes are connected by decreasing the natural dimenuendo between the notes. Sing through the consonants. (legato)
- Exaggerate the separation between the notes. (leggiero)
- Change the color of your voice.
That list is the approved list. Unapproved items you can also do:
- Begin the note under the pitch and go up to it. (scoop) This is much maligned but the work horse of expression in romantic opera.
- Connect two notes with all the pitches in between. (portamento, glissando or slide) If you can do this, stick it in anywhere you can.
- Interpolate high notes. This is a favorite on Idol.
- Sing a completely different note.
- Ornament where it isn't written.
- Manipulate your vibrato, take it out, put it back, make it bigger, make it smaller, etc.
Push the envelope. Discover the limits in singing. Maria Callas did every one of these things at one time or another, and we still can't get enough of her.
- Sing deliberately off the pitch, an essential feature of blues. I'm sure Maria Callas did this, too.
ps. Accidental going off the pitch is not allowed in any style. You have to do it on purpose.