Close up lenses do what they say. They give a small magnification by themselves, but also allow you to move closer thus making objects bigger. For more on them see my other notes in this
thread. They can be found very cheap, but those are made of low quality glass which leads to increased CA and unwanted reflections. There are more expensive higher quality ones, but of course they're more expensive. To keep at the same quality as the lens is normally while getting bigger images, also consider extension tubes.
Haven't used a TC myself, but do try to find a review for the intended lens+TC combination you're looking at. Many times I've seen it said that the quality might not be much better than you would get from doing a digital zoom in software with the lens normally.