Favorite value add for existing clients

Hi all, I'd love to know the biggest thing you love to go back and offer longstanding clients now that your Web ability has improved.

I've made sites for not for profit orgs for six years now and have a few constant clients who have been with me the whole time.

Is there anything you'd suggest i go back and offer them that's just obvious which you didn't do initially? For me it's been tweaking their responsive design and putting in decent caching plug-ins. Im a bit scared of getting into Google Analytics and SEO, but are there any good resources on the Bares Bones best practices that every site must have?

Thanks heaps, stay inspired 🙂