Which countries can be visited along with Iceland for a 14-16 days tour

Is your total trip including Iceland 14-16 days, or is the 14-16 days not including Iceland?

Are you going to Croatia, or is that a typo?

Have you already bought plane tickets?

Yes, Denmark and Norway are certainly easy to get to from Iceland, but there are lots of options. Keflavik airport has many more flights than just a few years ago. For a complete list of flights from Keflavik airport, here's there Wikipedia page: https://en.wikipedia.org/wiki/Keflav%C3%ADk_International_Airport#Airlines_and_destinations

Go to Google Fights. Choose Iceland to Europe, pick your dates. Select 1 way, then nonstop. Now you can see where you can fly out of Iceland to. You can even set max price.

I would look FIRST at the convenience and cost of flights, as James suggested.

Second I would think about the weather and what you'd need to pack for the possible locations.

In July we took a trip (from Los Angeles) to Barcelona, then London, then 4 days in Iceland. That was easy, flight-wise. But not so easy for packing, because the weather (and clothing needed) in Iceland was so different from the other locations. We had to carry (and pay for) a third suitcase on all 4 flights in order to bring the different clothing, shoes, and accessories that we needed in Iceland.