Hoi,
Start with your image alts, the most important being your logo. Add at least Spa Online in there.
"Zoek Spa's op locatie"
The anchor text isn't correct. You name the number of available spa's, but you should anchor them like: "Drente (19)".
Do a better keyword research and stem (Sauna, openlucht spa, dagje spa, massage in een spa, wellness centre, stoomkamer). Right now the pages aren't as targeted as can be.
Change pagetitle of /Spa-Map, walk through your entire site and stick with a consistent concise page titling scheme, where you check in Google keyword tool, how many visitors are predicted to come for such terms, and what the competition is on those terms.
Add plain text links on the /spa-map page that link to the inner pages. You want to start ranking for "Spa in Gelderland". The regional pages have subpar pagetitles. "Gelderland | Spa | SpaOnline" would be better to target that. "Gelderland | Spa, Sauna, Stoomkamers, Wellness" is another that would add value.
Write content for these pages, don't just list spa's. Write about Spa's in Gelderland, even if just 1 paragraph.
Build links in the many dutch directories. You have some links, and also some nice natural links, but startpagina's and all its clones are easy pickings.
Spa Online magazine works alright for content, I bet. You could add a little more focus on your listings and place a call2actions to "look for more spa's in your regions", "look for spa's that give these massages". Email spa's or use their account to ask for a more descriptions for their entries. user generated Reviews are good, but the content itself is a little lacking.
Use automatic xml sitemap plugin for joomla. Use analytics to find the keywords that bring you visitors and look to cement these keywords.
Try to add video, its a cheap way to rank in both youtube and with a videositemap on your own site. Find second-hand sites where you can place free listings. List your site as a service (Looking for a day at the spa? Visit our site).
Try to organize a competition. Find webmasters in your niche and offer them a day at the spa in return for a link and some promotion. Webmasters and social media loves free stuff and a prize organized with a spa (where u get the tickets for free or reduced prize in return for promotion).
Have a look at the guidelines and technical points of your website. Image alts "Click here for the slide". You think of hreview microformat and beg Google for a rich snippet
Standard more products in the /Gelderland -like pages, 8 is too little. 30 could be more appropriate, you are not gonna rank the secondary pages. Look at competitor sites like:
hxxp://www.yourw[remove]ellness.nl/spa/Gelderland
Look what they do correct to rank such a "poor" page in the top 10 and improve on that. Clue: Articles/content about spa's in gelderland. More and better links than your competitor, also to this very page. Individual product descriptions. Another top 10 competitor: hxxp://www.sauna-th[remove]alasso.com/spa-gelderland.htm Your site already screams quality over these kinds of competitors. You just need to take a little of their SEO medicine.
Try to remove dynamic variables from the url or at least use canonical meta tag.
There is really something to do on the SEO front. Consider training someone on the technical side of things or yourself to do a lot of this yourself. SEO can be expensive, too expensive to be worth it. With a little investment you can learn the basics of SEO and online marketing, two skill-sets that are nearly invaluable for a succesful website and online business anyway.