One of your first stops should be Harstad Tourist Office. Here, you'll be able to orient yourself and get some great ideas before you begin exploring.
This tourist information center in Harstad is well-liked by many travelers, so put it on your travel plans. Don't think that's all there is to do here, though. There are a lot more attractions waiting for you to explore after Harstad Tourist Office. Trondenes Historical Centre, Trondenes Church, and Grottebadet Waterpark are close by.