Hmm, chords...Actually, depending on the set-up of the instrument, continuous triple and quadruple stops can be played, provided that the bridge is low enough (you'll usually see this in baroque violins, I don't believe it's possible on other instruments). Also, triple stops can be played as a double stop + single string or single string plus douple stop, and quadruple stops can be played as double stop + double stop, which can be great, if the player is advanced enough. To hear this, listen to the beginning of Bach's Chaconne from Partita 2 for solo violin. Depending on what effect or mood you want to produce, harmonics can be awesome. Harsh tones can be obtained by using spiccato (bouncing the bow on the strings - the begining of Beethoven's Fifth) or sautille (where the bow strings actually stay on the violin string, but the stick bounces, and gives this harsh effect, though this can only be used in fast passages). There's ricochet, where the bow rebounds frmo the string in a single stroke (listen to William Tell Ouverture). Glissandi sound really bad if they aren't properly done, but smooth glissandi in the right places can be very pleasing to the ear. Then there are trills. You can make some interesting effects with the bow - playing on the bridge (sul ponticello) and over the fingerboard (flautando) (the latter is hard to accomplish if the bridge is low, and if you're playing on one of the middle strings). Then there's son file. Ask your girlfriend to demonstrate these. Then there are some modern effects, such as Bartok pizzicato - where the strings is pulled so that it snaps back to the bridge and produces this percussive sound. One can also play behind the bridge, making a scratchy sound (if you've ever heard this while listening to Piazzola, it's the violin). Then barriolage - alternately playing on two adjacent strings. There are lots of ways to attack the string, but you can only experience this if you actually listen for it in music.
Depending on who you're writing for, you will be able to use only certain techniques. For example, don't expect a beginning player to pull of sautille, ricochet or quadruple stops; artificial and doublestop harmonics can be reeeeeeeaaally hard; playing tremolo for a long time can be strenuous if the player isn't relaxed, too much sautille can harm technique, etc...
Ok, I hope I haven't confused you even more. You really have to listen and find out what feels right to you. Almost anything can sound ok on string instruments provided it's done with taste by an advanced player.
Wolf, as far as I remember, Mozart used col legno first, not Stravinsky. A lot of people think it's a strange technique, but it was used even then.