Hats
Hats are the fastest wearable there is. No sleeves, no seaming to speak of, and a finished object in a day or two rather than a month.
They are also the one project where gauge genuinely matters, because a hat two centimetres too big slides over the eyes and one two centimetres too small gives its wearer a headache. Make the gauge swatch. It takes fifteen minutes and saves the whole project. Here is how to measure it properly.
The granny square bucket hats are a good introduction to joining motifs into a curved shape — if the joins are what worries you, our guide to joining granny squares covers four methods and when each is worth using.