CHILDREN'S CONCERT -- Friday at 7:30. Singer-songwriter Billy B. performs songs about the beauty of the natural world. $2; maximum $5 per family; ages 2 and younger, free. Activity Center at Bohrer Park, 506 South Frederick Ave., Gaithersburg. 301-258-6350
HOWARD COUNTY CELEBRATION -- Saturday 8 to 4. Bird walk at 8, tree planting and trail maintenance from 9 to noon; picnicking from noon to 4. Free. Mount Pleasant Farm, 10520 Old Frederick Rd., Woodstock. 410-465-8877.
MERIDIAN HILL CLEAN-UP -- Saturday 9 to 2. Graffiti removal, weeding, trash pick-up and more. Free. Meridian Hill/Malcolm X Park, between Euclid, 15th, 16th, and W streets NW. 202-462-7275.
CONSERVATION FESTIVAL & CLEANUP -- Saturday 9 to 3. Stream and park clean-up, tree and shrub planting, hikes, fishing demonstration, children's activities and appearance by Gaithersburg City Mayor Sidney A. Katz. Free. Izaak Walton League of America Conservation Center, 707 Conservation Lane, Gaithersburg. 301-548-0150 Ext. 236.
"E-CYCLING" -- Saturday 9 to 3. Drop off your old electronics, batteries, cell phones, aluminum cans and tennis shoes for recycling. Free. Rock Creek Park's Carter Barron Amphitheatre, 16th Street and Kennedy Place, NW. 202-645-8245.
GREENBELT CELEBRATION -- Saturday 10 to noon. Plant native species at the Wetlands Educational Facility, see hybrid cars and tour the Public Works Department. Buddy Attick Park, 555 Crescent Rd., Greenbelt. 301-474-8004.
PATUXENT WILDLIFE REFUGE CLEAN-UP -- Saturday 10 to 3. Remove non-native invasive plants and other eco-friendly tasks. Free. National Wildlife Visitor Center, 10901 Scarlet Tanager Loop, Laurel. 301-497-5760.
MONTGOMERY COUNTY CELEBRATION -- Saturday 11 to 3. Environmental information, crafts, family entertainment, energy-efficient vehicle and bus tours of the recycling station, sponsored by the Division of Solid Waste Services. Field in front of 850 Hungerford Dr., Rockville. 240-777-6400.
FREDERICK FESTIVAL -- Saturday noon to 4. Information booths, food samples, crafts and more, sponsored by Community Commons and the Common Market. Free. Baker Park Bandshell, Second and Bentz streets, Frederick. 301-663-3416.
NAVY FESTIVAL -- Thursday 10 to 2. Environmental display booths, alternative fuel vehicles, music by the Navy Band Cruisers, children's activities and more. Free. Navy Memorial, 701 Pennsylvania Ave. NW. 703-418-3417.
ALEXANDRIA CLEAN-UP -- April 30 from 9 to 1. Help clean up the area along Four Mile Run Park; supplies, snacks and T-shirts provided. Free. Shuttles take participants to sites from Cora Kelly Elementary School, 3600 Commonwealth Ave., Alexandria. 703-838-4844.