TOUR HARBOR HOUSES
Fourteen homes in Federal Hill and Fells Point will be open to the public for the Preservation Society's annual Historic Harbor House Tour from noon to 5:30 p.m. Sunday.
The tour will highlight restored 18th and 19th century buildings, a renovated warehouse, a historic inn and the Captain Steele house in Fells Point. The Robert Long House, the city's oldest surviving house, and Preservation Society's headquarters, also will be open.
Tour-day tickets may be purchased only at the Federal Hill and Fells Point offices of O'Conor Piper & Flynn real estate agency. Admission. Information: 675-6750.
The Baltimore Museum of Industry, 1415 Key Highway, will be the host for the 1995 Maryland Engineering Challenge Competitions for middle and high schools from 9 a.m. to 5 p.m. tomorrow and Sunday.
Contestants will design a solution to an engineering problem, then test their solutions against those from other groups. A group of professional engineers designed the problems and will help the students with their designs.
The challenges range from building an electrodynamic suspension magnetic levitation train to creating a wooden bridge to carry weight while retaining aesthetic quality. Admission. Information: 727-4808.
BLOCK BY BLOCK
The Maryland Science Center is holding "Invention Adventure," a program that features what might be Den- mark's greatest export since Hans Christian Andersen: LEGOs, the plastic building bricks that captivate children and many adults.
Professional model designers from LEGO Systems Inc. will build a 10-foot-tall robot from today through the weekend, and you can watch them to pick up ideas and skills. The completed robot will remain on exhibit until May 29. A contest to guess the number of blocks in the robot will continue until the end of the exhibit. LEGO sets will be awarded to the first-, second- and third-place winners. Admission. Information: 685-2370.
Admission to the Science Center will be free Tuesday. All programs, including the planetarium and the IMAX theater, will be free.
The Baltimore City Life Museums are sponsoring "Bus Tour: Those Old Placid Rows," which will wend its way through 10 Baltimore neighborhoods and their rowhouses tomorrow and April 29. Admission. Information: 396-3279.
On Sunday, a Baltimore City Life Museums archaeologist will identify your artifacts, and a flint knapper will make arrowheads, arrows and bows using traditional American tools. The museum is at 800 E. Lombard St. Admission.
HELP CLEAN RIVER
If you want to do something to mark Earth Day, help clean the Middle Branch of the Patapsco River. The Middle Branch is the "unseen" waterway that also feeds Baltimore's port and the Chesapeake Bay. Hours: 10 a.m. to 3 p.m. tomorrow. Place: Off Russell Street on the south end of the parking lot at Oriole Park at Camden Yards. Information: 962-3500.