Core exercises

I don't know if these links will help you to your goal (it's pretty aggressive), but I think they'd be a good start. They are to articles with links to exercises.

http://www.quickanddirtytips.com/health-fitness/exercise/how-get-flat-stomach
http://www.quickanddirtytips.com/health-fitness/exercise/flat-stomach-exercise-routine

You also may find some suggestions here: http://www.bodybuilding.com/exercises/ (just click on abs, and then filter it so that only bodyweight exercises are shown).

I would also suggest when you're sitting at a desk or walking, to intermittently contract the stomach (like you're bracing it for an impact of some kind) and hold for a few seconds, and then pulling in the stomach (like guys who walk past you probably do to hide their beer bellies) and hold for a few seconds -- for 10-15 reps. It's not much, but it's a little tiny bit something. You can do it throughout the day, maybe 8-10 times or so.

Keep in mind that diet is ultimately the most important thing for having definition in your core. Good luck to you!

I'm giong away in just over 2 weeks so would love to tighten my core/abs a bit before then (may even venture into a bikini).

you're wanting to change how your body looks in two weeks, not just strengthen your core. reducing bodyfat is what it's going to take, and while ab workouts will help with definition, reducing overall body fat percentage is the only way folks are going to see those abs.

2 weeks is pretty aggressive.

You can't really do an exercise to "tighten and tone" your core. You can do exercises to strengthen the core, though, but to get the slimmer "tighter" look in the abdominal area it takes time patience, and a caloric deficit. You can't spot reduce. You could do 10,000 planks per day, but if your body decides to burn fat from a different part of your body (which it usually does), it will.

I would recommend a mix of cardio and compound lifts that include core exercises.

Good luck!

^ This.

Thanks for your suggestions

I know I'm not going to get washboard abs in 2 weeks, but surely I'll get some tightening in that time and be able to see a difference. I'm not really big in my stomach area and just want a little more definition but don't have hours every day to do pilates/yoga classes etc so was asking for suggestions along the line of the squat challenge which I have tried and seem results in my legs and butt.

I run and cycle regualrly so do plenty of cardio, but wanted something to work on my core.

Look, I don't know how long you have been doing MFP, but a lot of us are doing it for months or even years. If there were some quick fix diet or exercise that any of us could do to shrink, tighten or define our midsections, or butts in two weeks, we would all be doing it.

You can only do so much in a very short period of time to change your appearance. Even given a longer period of time, your YMMV based on genetics and just how much you are willing to sacrifice to have a great body. As you said in your post above, you don't have "hours a day to do pilates/yoga, etc" And that is fine, but basically, you are looking for a short cut, and there are no short cuts.

Finally, I am not a believer in these 30 day challenges, . They strike me as the exercise equivalent of the 7 day cabbage soup diet.