Well, you can take the
Metro Green line out to Greenbelt Station. From there you take the
B30 bus from the station to BWI. Then you take the shuttle bus from BWI Station to North Linthicum station and take the
Blue Line light rail to Hunt Valley.
For the Light Rail, you want to buy the Day Pass for $3.50, as sometimes the one-way expires before you reach HV and the transit cops are pretty dilligent about checking for fares. The B30 is like $2.50 (cash only). And Metro depends on how far you are going (DCA to Greenbelt is probably around $4. Metro Center to Greenbelt is probably around $3.50).
DCA/Metro Center to Greenbelt is about an hour or so. The B30 can take upwards of 90-120 minutes depending on traffic (I took it on weekends so traffic was light).
Travel time from BWI to Hunt Valley was about 90 minutes (including the Yellow Line to Blue Line transfer which is no longer required), but I do not know how long the shuttle bus adds. Check
the schedule to see.
Consider renting a car. I got a one-way Full Size DCA-BWI on a Sunday for $25 from Hertz. Driving takes about two hours or so and is all highway.