Most people I know in the DC area grew up in other parts of the country, many of them in places like Boston, New York and Chicago that have cooler climates than the Mid-Atlantic. The weather there takes longer to warm up enough for outdoor activities and there's more time before summer moves into the dog days when it's hard to do anything outside without sweating.
Then they move to DC and keep that same mental weather calendar and wonder why they're practically dying of heat stroke on their Memorial Day hike because it's 95 and humid. But Memorial Day was always perfect for hiking in the Blue Hills!
Don't let the nice days slip past you. In the early part of the year, watch for those 60 degree days in March, then keep an eye out for them again in November - the times of the year when many people wrongly assume it'll be too cold to hike are in fact the best days to get outside. All you need is a little sun & a little warmth and if you're on a good hike, you'll be glad it's not warmer.
If you're looking for a really close, easy trail with great views, you can't go wrong with Great Falls Park. The trail to the north of the Virginia parking lot goes for miles, providing a good walk along the Potomac River and a chance to spot Great Blue Herons, Egrets, and maybe even a Bald Eagle if you're really lucky.
And on those first days of warmth, you can get a rare look at Great Falls' angry side. The melting snow can combine with spring rains to turn Great Falls into raging rapids:
One tip - the parking lot can fill up fast on really nice days and during prime fall foliage, so go early.
TheGreenMiles.com's Take a Hike Week
Tuesday: Old Rag MountainWednesday: Catoctin MountainThursday: Scott's Run Nature PreserveFriday: Virginia's Great Falls North