Re: Specific question..
The only real tip I have is - improvise.
I make melodies that usually have vocal phrasing, which naturally contain different rhytmic values (4ths, 8ths, 16ths, triplets, whole-note, etc), aswell as the important use of pauses/silence/breathing. Glissando and vibrato can also make things more interesting.
Sometimes more "extreme" intervals can liven up a more "arpeggio-like melody".