Hi! We have an 8 hour layover at Schiphol on March 14th - it looks like the Tulip Festival doesn't start until the 20th, but does anyone have advice on where to go (without a car) to see some a week early? It looks like Keukenhof doesn't open until March 20th.

You almost certainly won't see tulips on that date, at least not fields full of colour. They flower earlier than they used to but not that early. Other bulbs, such as daffodils, crocuses and maybe hyacinths will be flowering then hopefully, but where you would see them close to Schiphol I couldn't say.
Hopefully menachem sees this and knows more.
You need to work out the logistics of it all as well, depending on where you are flying from and to, to be sure you are back at the airport in plenty of time to got through passport control if leaving Schengen and security.

No tulips on March 14th

Hyacinths and daffodils may be out, as hetismij wrote, and there are a few fields of those between Schiphol and Leiden, near Lisse and Hillegom (flowerbulb country). We planted tulip bulbs on the balcony and they just started growing their stems out, they'll be out at the beginning of april, I think.