In Denmark, tipping isn't common - all service bills that you receive already include gratuity, and it is unnecessary (but always appreciated) to add a tip in Denmark.
If you're at a restaurant, don't leave without giving at least a small tip. Check the menu or the final bill for "tip included / not included" to make sure how much you should give. Or, go the easy route: If you feel you've received really good service, the appropriate amount to tip in Denmark would be up to 10% of your bill.
You can leave this tip by rounding up the total amount. And you'll see a lot more smiles from staff! You can even offer your hotel service person a couple kroner here and there.
For example, if the bill for your dinner is 121.60 and you received excellent service, it would be appropriate (but not expected) to give them a total of 130.- in the local currency, Danish kroner.
In Denmark, tipping isn't even needed for taxis - their final charges already include gratuity as well. If you come across a service that you'd like to give a tip for, though, offer a small amount of around 8-10 DKK, or give this tip in form of "rounding up" to the nearest even amount (e.g. for a charge of 72.40, simply give 80.00 even).