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Internet advice from a random stranger: Like it or not her friends and family will judge you poorly if you don’t get her an engagement ring, even if they don’t say so explicitly to either of you. All marriages go through tough times and you absolutely need them to be supportive of your marriage when that happens rather than say and he didn't even get you a ring
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For counterbalance, I proposed and bought my OH an engagement ring when we were travelling. It was a very cheap (but nice looking) placeholder. When we got back they decided there was no point in spending money on another "proper" ring, because a) they already had the actual ring I proposed with, and b) because they wear a wedding ring every day not an engagement ring.
Others might disagree, but I think the whole engagement ring arms race has got a bit ridiculous, and in its current form is another eg of a negative Americanism.
I'd be tempted to get some sort of token ring, but if you didn't give a ring when you proposed I'm not sure it has any meaningful value.
Tl;dr I think a watch sounds like a great idea.
@Howard has it 100% right.
Seriously, buy her a watch as a wedding present and get the engagement ring for your engagement.