Cubs games are another defining part of the Chicago experience.
Cubs games are pretty regular (during the regular season), and tickets are easy to get. They can cost anywhere from $10 to $50 depending on what seat you get. Inside the Wrigley field there are tons of hot dog vendors (selling Chicago style hot dogs of course, but also pizza, burgers, etc). There are many restaurants and bars around the stadium if you're hungry when you leave, but I'd recommend going a couple El train stop away or else you'll get caught in a mad rush.
The games are plenty of fun, there are pretty regular give aways at the gates and from surveys, etc. I got a free little Snoopy doll (Given at gates) and a free fleece Cubs blanket (bank survey). You can also bring food and drinks in if you want, just not a huge cooler (you could try haha).
Transportation to the field is easy enough, there are major thoroughfares around it, but parking is rough. My suggestion is to park several El Stations away, and to ride a bus, or catch the El, or just walk (the nearest stations are two blocks away from the field.
The games are pretty fun, even though the Cubs haven't been awesome recently. The games are never jam packed, but they can be busy. It is easy to get a large group in, but you should get tickets ahead of time.
All in all Cub's games and Wrigley field are a great way to relax and enjoy the summer and fall nights. With the sights and sounds of a ball park, especially one with such history, nothing can go wrong (except rain).