I think as a web designer you could look into offering may types of
SEO services. At the end of the day
SEO has no trick; only hard work. As such I don;t think the services you can offer will be different to what and
SEO expert may offer.
Have you though about offering new content services. If many of your clients belong to a specific industry then maybe you can look into creating combined RSS content that you can offer specifically to your clients. As long as you do not syndicate your content to too many other websites and keep it specifically for your clients each one of them will still be able to get some benefits to their search position due to new content on their website.