I haven't had to do it but I would imagine that picking out an engagement ring that your betrothed will wear for the rest of her life might be a teensy bit stressful. Just a hunch. One way around this is to let her choose her own ring, that way she gets exactly what she wants and the pressure is off. Of course, you don't want to show up to the engagement empty-handed, which is where these necklaces from Giles and Brother come in.

Their Single Rhinestone Pipe and 10 Strand necklaces each feature a ring so while you're still giving a nod to tradition, it's piece she can wear for years to come (and to all of her pre-wedding events).

Long, pendant necklace alternatives that also feature a ring.

Or if she's stolen your heart (I can't help it, I'm knee-deep in love over here), one of their Heart Locket necklaces might be fittingly symbolic while their simple Tiny Key necklaces let the world know she has the key to your heart (I warned you). Both of them are nice because they're a bit longer so they could be worn casually, on a daily basis.

Heart lock pendant necklace with a matching key as an engagement ring alternative.
They also have tiny initial charms, locket earrings and a whole lot of other unique finds that would be meaningful yet also wearable.