Things to keep in mind:
Both MARC and VRE are Mon-Fri only operations. There is no weekend service.
VRE does have about 4 reverse commute/midday turns on the Manassas line (NS) that you could probably ride. Take a look at the schedule, and you will see which DC departures turn back to Washington at the end of the run in Manassas.
There is an early afternoon train to Fredicksburg on the CSXT (ex-RF&P) line there, but apparently it is a deadhead run back to DC (its train 301 out of DC at 12:55 pm, but there's no comparable return trip). You'd have to take Amtrak back, and its a long wait for Amtk #80 at 3:45 at Fredicksburg.
http://www.vre.orgfor schedules.
VRE I know has a real variety of equipment. Poke around on their site for consist info...you can actually find out what your train is supposed to have before you ride!
MARC is probably only doable on the Penn and Camden Lines. The Penn Line runs roughly hourly all day on weekdays, so there's always return options from Baltimore. Probably best to ride a Camden Line train from DC to Camden Station, Baltimore, and then hop the light rail from Camden to Penn station. The light rail is also an interesting ride, the northern part being ex-Northern Central (PRR) ROW, and the south end is the former Balto. & Annapolis ROW (old interurban). I believe, however, the norh end is currently shut down for double tracking. The south end is still an interesting ride...esp. how the line that was once an interurban is essentially re-creted in modern form (just that it only goes to Glen Burnie, not Annapolis).
Hope you have a good visit there. Also, don't forget that some of the Washington Metro lines parallel railroad lines, and offer good chances to railfan there too.