Me, I love carrying spare parts. On a recent trip my partner's gear cable failed. I pull out the spare, 5 minutes later we're going again. Basically, all the spares I carry are from previous experience:
- tubes
- spokes
- derailleur hanger
- chain links
- derailleur cable

None of those are particularly heavy.

And seconding everyone who said to bring less clothes. Me: one pair of knicks, one jersey, one pair of lightweight zip-off pants, thermals (top/bottom), rain jacket. Plus usually three pairs of gloves, because I JUST DO OK?