The Susquehanna River from Havre de Grace to the Conowingo Dam is wide, slow moving, and not very heavily developed on the southern side. The river draws a lot of fishermen, but does not seem overly crowded even at heavy use times. The river can be paddled all the way to the Conowingo Dam, but does get quite rocky above Robert Island (the large island just upstream from the I-95 bridge). I've been there several times, and have always found it to be a relaxing destination. I typically put in at the municipal launch area in Havre de Grace and go left up the river, although those wanting to paddle the Chesapeake Bay can do that as well.
If you don't want to take your own boat, rentals are available at Starrk Moon Kayaks just down the street from the launch area.
Full range of accommodations are available in Havre de Grace.
The state park launch area has a $5 launch fee.
The Havre de Grace launch area has a $5 fee on weekends, but is free during the week.
Take the Havre de Grace exit off of I-95. Follow the signs into town. The municipal launch area is at the north end of town near the railroad bridge.
Public launch area is located at the north end of town just north of the railroad bridge. If you have trouble finding it, look for the American Legion (on the water) or Starrk Moon Kayaks (just south of the launch area on the opposite side of the street).
Those wishing to launch further up river can use the state park boat launch on at Susquehanna State Park. It is accessible from Lapidum Road, which will be on the north side of the road after you exit off of I-95.