Radishes are always a good choice! They are very straightforward in growing and mature very quickly - 4-6 weeks after sowing, they can already be harvested. So you can always exploit a free gap in a vegetable patch in between cultures.
By the way, radishes don't always have to be small, red and round. There are also elongated, pointed, red and white or yellow and white varieties, and of course the larger white and black ones.