As your lawn will probably a mix of various grasses, a general purpose grass seed will be the best option. Dont get cheap 'hard wearing' rye grass as it will send up tough seed heads which can be a pig to cut. Scratch the bald patches with a wire rake a few times, sprinkle the grass seed on and scratch again a sprinkle of fine soil will help. At this time of year germination will be 10 days or so and you are away - dont let it dry out!!!
As for the dips, for small dips,soil/sand/seed mix will be oK, but if the dips are fairly deep, it is better to put a board down the middle of the dip, cut a slit along the length with a good sharp spade, then at right angles cut across the dip every 9" or so. You will then be able to slide the spade under the turf(about 1" down) and fold it back(like unzipping it) place in enough soil so that the turf is a little proud when folded back. Beat with the back of the spade - job done.