I'd like to give you a couple of exercises that I found to be fairly helpful when strengthening my core, but first, I want to point out that two people made a few REALLY good points on here that you should not overlook simply because they did not list quick and easy exercises for you:
#1. Core work has it's place and time. Just like your running workouts, a core workout should be specific. You should know why you are doing that specific exercise and how it will benefit you.
#2. Doing a bunch of core work in the same plane of motion (i.e. 500 crunches and 10 minutes of planks) is not going to help you; it could very well cause negative changes in your biomechanics.
Core work should be periodized. Just as in running, your system will adapt to what you are doing. I like to swith my core routine slightly every 4-6 weeks. I will make 2 different workouts of anywhere between 4-8 exercises. It all depends on what muscles are being worked. As a previous poster said, core doesn't include just your abdominals. The easiest way to understand what your "core" muscles are is just picturing your body without the legs and arms (I tend to include the pelvis in my core workout). Your abdominals, olbliques, upper and lower back, and pelvic muscles. I will make a working A and B and then rotate them every 48-72 hours. There is not a magic number of days/week that will give you a better core. You need to be able to complete the workout to see the benefits. So if you are tired one week and only complete it twice, that's ok. If you feel good the next week and can get in A/B/A on Mon/Wed/Fri then great.
One other tip I have for you if you want to get your "Bang for your Buck" is to do the majority of your core exercises at tempo. What is tempo? Core is considered a form of resistance training. Even without a medball or weight, you are resisting the weight of your own body. The "tempo" refers to the time you are resisting weight. I choose to use a count of 3-1-1 when performing core exercises. I truly believe this was the biggest key in helping me get stronger and actually keep track of my progress. To do a rep with a 3-1-1 tempo, an example would be:
Crunch:
"1" = Count of 1 to RAISE your core up
"1" = hold and contract your core muscles for a count of "1"
"3" = SLOWLY lower your body back down for a count of "3"
The lowering (eccentric) phase is the most important phase; you are going to be recruiting the most muscle fibers as you fight the weight of your body on the way down.
The ONLY time I do crunches is when they are at tempo. I use a yoga ball to increase the intensity of the exercise and I will do anywhere from 2-3 sets of 12-25 CONTROLLED crunches @ Tempo. I say 12-25 because the first time I started this type of crunch it was really difficult. I was tired after 12 of them! I'm now doing 2-3 sets of 25 depending upon how I feel that day. It's a great way to build abdominal endurance and use more stabilizers (especially on the yoga ball).
I can give you a list of other exercises and sets/#'s. But you should realize there are no MAGIC exercises that are going to be best. It's the quality of your workout. Just like running, core should be quality over quantity. Too much of any type of core exercise is going to be bad. Not just crunches. But if you have a healthy balance, most of any type of exercise is ok to do.