The salad radishes are very easy to grow and many of them are very fast when conditions are right. Early Scarlet Globe is one of the best earlys. Rudolf Radish (Raphanus sativus 'Rudolf') is early with very compact tops. Sparkler Radish (Raphanus sativus 'Sparkler White Tip') a little longer and trickier to grow (bolts and splits when conditions are not suitable) One of the prettiest tho.
If you willing to wait a week Other colors are great. Radish (Raphanus sativus 'Amethyst')