I was recently visiting a friend and, as we toured around her city, she would occassionally pull one of these from her purse and dust it all over her face:
Peter Thomas Roth Instant Mineral SPF--a transparent SPF 45 mineral powder in a convenient, retractable brush applicator. The powder is ultra-fine and leaves a layer of sun protection. You can dust it right over makeup, so you can touch up your SPF all day without having to deal with a liquid product. Plus, it's small enough to carry in a small handbag. My friend highly, highly recommends it, so I'm picking one up this weekend.
No more excuses for not wearing your SPF, friends.
(Photo from: Sephora)