There are some keys that just beg to be riffed on. The key of D is especially easy because you can hold your chords and still reach all the notes of the scale. Here are some ideas to get you started.
You'll notice this tab doesn't start with the root note (I've labeled it on top), but any note in this scale will work. It's easiest to reach these notes when playing your open D Em G A and Bm but don't limit yourself to that. Below the scale I've listed some song lessons that are conducive to practicing your riffs. If you aren't a Worship Artistry member you can sign up for a free account here.
Practice your technique with these songs