Thick card
A laser printer might do the job, but you have to be careful what brand and model you get.
My old, old, Apple Laserwriter NT did an awesome job of printing on anything I put through it... it never let me down, envelopes, card stock, fancy papers, it never mattered. I never had jams and it never complained or smudged or refused to print. It had an excellent paper feed mechanism. It still works, by the way, even though it's ancient. The only problem is, it's only 300 dpi.
I replaced it with a higher resolution GCC Elite1212. Good printer, reliable, also old and still working fine, except the paper feed is temperamental. I've never gotten good results with card stock, especially heavier card, and it often urps on envelopes too. It's fine for plain paper, but I don't recommend it for anything else, even if you do the tray tweaks that GCC recommends, the printer is finicky.
My ip5000 and ip4600 seem to be fine with paper of different weights, but I'd be nervous to use them for certificates, because the ink is not waterproof on regular papers and a certificate would look kind of odd on photo paper unless it maybe had a matt finish. The ip4600 is very fussy about paper trays (see my other posting).
You can run canvas and watercolor papers through Canon inkjet printers, so they are pretty versatile. T-shirt decal paper can cause problems though. If a tiny bit of that sticky paper tears off the edge and gets caught inside, the rollers don't work right any more and paper feeds crooked.
My printers tend to last a long time, even though I use them regularly, so I don't have experience with many different brands, but maybe the above experiences can help in a small way. Good luck.
