For a day-to-day moisturizer, any one with a SPF of 15 to 30 will do. If you are going to be outside for longer periods of time, especially during the middle of the day, I recommend using sunscreen in addition to the moisturizer, with a SPF of 45 to 50. Make sure your sunscreen is labeled "broad-spectrum" as well; you want protection from both the UVB and UVA rays. The FDA actually just recently passed new regulations that require manufacturers to label their products (which will include cosmetic companies that have moisturizers with SPF) "broad-spectrum." Before these new rules, sunscreen manufacturers were only required to report the SPF value of the UVB rays (or burning rays) and not the UVA rays (which are responsible for actually aging the skin and causing damage at a cellular level).
Also, if you are looking for something a little more natural, try a mineral sunscreen. These usually contain zinc oxide or titanium dioxide (or a combo of the two). They come in the lotion-type products or powder based. The powder based sunscreens are great because many of them can act as a setting powder and sunscreen, plus you can re-apply them as necessary. Bare Minerals makes a loose powder with SPF 25 and a pressed powder with a SPF 15. Hope I could help!