In my opinion Soller is almost a tourist trap. It is not an ugly town, but it's not a nice place.
Unique Suggestions: Do some shopping, have a rest or a stroll.
Fun Alternatives: You coud go to Biniaraix; a small village 1,5 km far from Soller. From there you can continue through a path to Fornalutx. This is a beautiful village four km. from Soller (see pics).
Another idea; catch the tram to Port de Soller; a seaside resort not far from Soller. From the tram you can enjoy a beautiful landscape. You will see lots of oranges and lemon trees.