Ocean City is a pretty big ocean town. Long boardwalk, lots of bars, restaurants, tee-shirt shops and other beach town ****. Ocean City itself is very crowded, lots of cars, lots of traffic, hard to find parking. There is an amusement park on the south end of the boardwalk. In the Ocean City area are some kid amusement places, movie theaters, mini golf, malls, etc.
We usually stay further north in Delaware. Before we had kids the wife and I would usually drive down one afternoon to Ocean City to grab dinner and walk around the south end of the boardwalk one evening. Usually one night of that was more than enough for us. Lots of teens and early 20's kids. Lots of alcohol induced stupidity. Lots of cussing and fighting. Not very family friendly in the evening or night time hours.
If you get there and decide you are bored or not liking what is available to you, its a short ride to Fenwick Island (15 minutes), Bethany Beach (30 minutes), Dewey Beach (35 minutes), Rehoboth Beach (40 minutes) and Lewes (an hour). Each of these locations is uniquely different and you should be able to find something to entertain you in one of these areas for 5 days.