There are lots of great trails to hike and bike in the Ocean City area. There are also several kayak tours around the nearby coastal bays, rivers, and swamps.
HikingFor hiking there are the two obvious choices, the beach and the boardwalk. There are also a couple of interesting historic self-guided walking tours of downtown ocean city. You can get the guides to these two tours here and here. Ocean City's Northside Park at 127th Street has several walking and biking trails.
Most of the biking paths below also accommodate hiking.
Biking From mid May to Late September bikes are allowed on the 2.25 mile boardwalk from 2am to 11am. Outside of this time they are allowed anytime.
There is a bike lane for the entire 10 mile length of OC on Coastal Highway, but you will be riding in a lane shared with buses and turning cars.
However, if you drive (or ride) up to Delaware, traffic gets much lighter in Fenwick Island
and north and riding on Coastal Highway there is much less stressful. You can ride pretty far north on a bike Lne, at elast to Bethany and probably much further
You can rive across the first Route 90 bridge and park at the Nature area on
the Isle of Wight. Take St Martin's Neck Road (the road crossing Route
90) and ride into the Lighthouse Sound area and along the back bays.
You can take the Route 50 bridge to the park and ride lot (left at the first
light after crossing the bridge). Park there and then ride down Route
611 towards Assateague. There is a bike lane most of this route. There are lots of great side streets to ride on
there too.
All of these Routes are safe and scenic. They also let you ride as short or long a distance as you desire.
This site lists 7 cool bike trips to and around OC with maps and directions. The longest is 144 miles from Baltimore to OC and the shortest is a 10 mile trip to Assateague. You can easily modify these routes to fit your needs.
Kayaking and Canoeing Ayers Creek Adventures offers some great kayak and canoe tours of the waterways in the area. They will often customize a tour to meet your group's needs.
Have fun in OC!