Searching for advice in Reykjavik

Hello everyone! Halló allir!
We are heading to Iceland in February for a Wednesday to Sunday trip and will spend 3 nights in Reykjavik.
I'm searching for advices about:
- nice things to see a part from the most famous attractions;
- nice and possibly not too much expensive cafè and restaurants (we already know that Iceland is a little bit expensive for our standards, but maybe we could avoid some tourist trap, plus I have to try Hakarl and so I'm searching where is the best place);
- good spots around the city for seeing northern lights. I have found Grottuviti, but maybe there are better places.

Plus, I was thinking to ask some opinions about our program:
- we arrive in Wednesday morning/noon and go to Reykjavik, spend afternoon there;
- Tuesday, drive to Hof, on the road visiting Seljalandsfoss, Skógafoss, Sólheimajökull, Vík í Mýrdal, Svartifoss;
- Friday excursion in the morning at Vatnajökull and Jökulsárlón; in the evening drive back to Reykjavik;
- Saturday visiting Þingvellir, Haukadalur, Gullfoss.
- Sunday return home.

Consider that we are going to wake up very early in the morning and have no problem in doing long road trip driving. But maybe this program can be adjusted in better ways.

Sorry for the long post, but I prefer to have everything in same place. Thank you in advance!