I are on a Non-O Marriage Visa. I have done it outside so I need to leave for border every 90 days..

But this is my situation, I leaving Thailand 18th of May, and my 90 days is on 14th of May. So I just wish to extend for 7 days.

What kind of documentations is needed? Do I need to bring my marriage certificat etc.? Or can I just show up with 1900 Bht, fill in the form they have and take the pictures there at Chaeng Watthana immigration office in Bangkok?

You will need to take with your passport, completed application form TM7, recent passport sized photo, copies of your passport photo page/visa/arrival stamp + TM6 departure card. Fee 1,900 baht. Expect immigration will also require sight of air ticket out of the country within 7 days.

http://www.immigration.go.th/nov2004/download/tm7.doc

(print form double sided on single A4 sheet of paper or obtain/complete at immigration)

Or you could do a border hop and get 15 days at the border, it will take all day at CW and cost the same as these people http://www.thaivisarun.com/ you may need to show a ticket out if you're reentering without a visa though.

It most certainly should not take "all day" to obtain 7 day extension at Chaeng Wattana. Maybe an hour, maybe less; depends on number of applicants. And there is no need to show a ticket out if entering the Kingdom on visa exempt entry at land border crossing.