Well with the hydrangeas it depends on whether you bought tropical or hardy hibiscus. They fall into two major categories. The tropical types have flowers with petals the curve back with large stamens. The hardy types can have huge flowers, but they are more like dinner plates with short stamens. If you have tropical hibiscus, you can try to overwinter them indoors in a sunny window. If you have hardy hibiscus, get them in the ground as soon as possible and cover them with a bit of mulch after they go dormant in the fall. They will not survive indoors. As far as dahlias go, leave them alone till frost lightly blackens their foliage. Then, dig them up, wash the soil away from the tubers and clip away the foliage. Then, you can store them in a cardboard box filled with dry vermiculite, perlite or sphagnum moss. Keep them in the dark and keep them cool till spring when you can replant after frost danger.