Resistance was coordinated from England via radio using code words. English planes droppes supplies (explosives, weapons) and also parachuted specialists to train resistance, mostly danes who had fled to England.
railroad tracks were blown up, factories and garages that served the germans were burned. On a small scale sugar was put in petrol tanks of german vehicles to ruin the engines. Informants and some collaborators were assassinated.
BUT it wasn´t really a big deal until late 1943, when things were going against Germany and the occupation became more harsh. Biggest danish contribution to the allied cause was the big danish merchant fleet, which sought allied ports when Denmark was attacked, and served in the Atlantic convoys.
Finally Denmark was the occupied country where most jews survived, 99% managed to flee to Sweden, aided by friends and the resistance. I think only about 50 of 7000 died.
In the big picture Denmark was an asset for the germans: Lots of food was sold to
Germany and thousands were sent there as laborers. Also thousands volunteered to fight for the germans against USSR, Some in "Frikorps Danmark" (about 1200 attached to the SS Totenkopf division in the Demjansk pocket), some in "Regiment Nordland" in the Wiking SS Panzer Divison (took part in the final battle for Berlin), some spread out in other units. (According to german law foreigners couldn´t serve in the Werhmacht, so all foreigners were in the SS) I have seen the number 8000, but think it might be lower, the number could include paramilitary units like the Schalburg Korps - a bunch of bullies to beat up protesters and punish the population for resistance activities.