Strings
All harps require strings, so any harp player will be able to make use of a gift of harp strings; however, harps with pedals and those without pedals use different types of strings. The strings on non-pedal harps are kept at a lower tension, so those types of harps may be strung with nylon composites or nylon strings. Gut strings can be strung at a higher tension, so they may be used on pedal harps. Wire strings are strung using different nails in the harp than other strings; they're ordinarily only used on folk harps.
Tuning Supplies
Each harp comes with a tuning key. The harpist turns on a chromatic tuner, plucks the string, reads the tuner and uses the tuning key to adjust each string until the chromatic tuner reads it as playing in tune. Tuning supplies that would make good presents for a harp player include a chromatic tuner and a replacement tuning key. The chromatic tuner is also useful for tuning any instrument, not just the harp.
Metronome
Harpists often play and practice alone. A metronome can be useful in helping the harp player learn and keep to a new piece's beat. Metronomes are set to keep a particular tempo; they emit a sound or light on every beat of the tempo. Consider whether the player would prefer to see a flashing light on the beat or hear a sound on the beat. Learn how long the metronome runs once it's wound or charged.
Sheet Music
Harp players usually learn new pieces by reading sheet music. When choosing sheet music for a harp player, buy music that's appropriate to the player's expertise level and type of harp. Compilations of harp sheet music that are geared toward a specific occasion or a specific musical tradition are also common.