Get some sunshine. Vitamin D keeps bones strong.

The human body does not have a requirement for vitamin D when sufficient sunlight is available. However, vitamin D does become an important nutritional factor in the absence of sunlight. This is because the
body produces vitamin D when it is exposed to sunlight.
Why do we need Vitamin D? It is necessary for your body to absorb and
utilize calcium, which keeps bones strong. So get your 20 minutes of sun a
few times a week. A vitamin D deficiency can lead to osteoporosis in adults or rickets in children.

Other sources of vitamin D:

Cod Liver Oil, 1 Tbs: 1,360 IU
Salmon, cooked, 3 1/2 oz: 360 IU
Mackerel, cooked, 3 1/2 oz: 345 IU
Sardines, canned in oil, drained, 3 1/2 oz: 270 IU
Milk, nonfat, reduced fat, and whole, vitamin D fortified, 1 c: 98 IU
Dry cereal, Vitamin D fortified w/10% of the recommended daily value, 3/4 c: 40-50 IU (other cereals may be fortified with more or less vitamin D)
Liver, beef, cooked, 3 1/2 oz: 30 IU
Egg, 1 whole (vitamin D is present in the yolk): 25 IU