in general excercise routines, small weight and many repititions will tone, whilst larger weights with fewer reps leads to muscle growth, so avoid lifting large weights on machines etc. swimming is ideal for what you want. if that's not possible good old sit-ups work too, however when doing sit-ups, ensure there is no bending at the hip joint as this lessens the effect.
when doing a sit-up, the first part of the body that bends is the stomach and lower spine, if you carry on up you will notice that the hip joint bends next. this bit is pointless and won't tone your stomach so you keep the sit-up within the range of the abdomen only. try it