You know, donut shaped. ISTM that the rivers would run towards the center, rather than towards lower altitudes. Depending upon the proportions of the planet, anything on the surface of the "hole" in the middle of Planet Toroid could experience negative gravity and float towards the center of mass in mid-air (or mid-vacuum). It would get crowded there after a while. The geometry might be WAY different. On some toroiplanets significantly shorter routes could be had by veering towards the center, if you were equipped for any nasty gravity effects. Only the bravest and most intrepid would stray TOO far from the outer circumference. What else?