Aside from talking to them daily, you have two gift options:
- Foreign fruit. This is a lot slower but it's the best option if you want to avoid altering their furniture.
- Iron wall lamps/Hanging terrariums. This is by far the best item to give since it raises friendship very quickly. Although, while they will not display it in their houses, there is a small chances of them removing whatever wall furniture they have. Also, it's a litle resource-heavy (clay & iron nuggets) if you're going to give it daily.