It's entirely possible to visit Bucks County as a day trip from Philadelphia, but a few days more allow you to sample many of the area's pleasures.
If You Have 1 Day
Start in Doylestown at Fonthill, the fantastic mansion built by local Renaissance man Henry Chapman Mercer. As lunchtime approaches, wend your way along U.S. 202, stopping at any antiques shops displaying wares along the roadside, usually indicating they're open for business. Have lunch in New Hope, take a quick stroll through town to check out the eclectic boutiques and gracious historic buildings, and then head north on River Road for a stirring drive along the Delaware River, returning south on the New Jersey side for dinner in Lambertville.
If You Have 2 Days
At your starting point in Washington Crossing, you can see where George Washington set off across the river with his troops on that fateful Christmas night in 1776. Then drive up River Road to New Hope and have lunch, perhaps outside. In the afternoon check out the town's stores and galleries or head across the river to the antiques shops of Lambertville before enjoying a tasty dinner on either side of the river. On Day 2, take a walk or bike ride along the canal, and then head west to explore Mercer Mile in Doylestown.
If You Have 3 Days
Make your base in New Hope or somewhere nearby. In season—roughly from May to October—take a stroll around either New Hope or its sister, Lambertville, and continue the strolling (or biking) on the river's bucolic towpath. On Day 2, head west on U.S. 202 to visit the museums on Mercer Mile in Doylestown, stopping along the way at Peddler's Village in Lahaska. Stay in Doylestown for dinner.
History buffs can fill their plate on the third day by starting out with a tour of the pre-Revolutionary village of Fallsington and Pennsbury Manor, William Penn's country retreat, both near Morrisville in the southern part of the county. Drive north along the river in the afternoon, and you'll end up at Washington Crossing, where a walk through the glorious parkland can easily occupy the rest of the day.