That's what the OP said, not the key of D major/minor or the relative minor or anything else and I hoped s/he was coming back to explain why. It is, of course, a jolly good idea to attempt tunes in many keys at will in order to practice counterpoint and test how well in tune your instrument is, as Bach realised in the early 18th century with the Well-Tempered Klavier. That, however, was not the question, which was about tuning not pitch

A different way of phrasing it might have been 'What tunes are best/most easily played in open D?'

Or (as I said earlier) 'Which tunings can you fall back on if a string gets stuck?'