Yes, you can. The abdominal muscles should be getting a workout throughout your day just by stabilising your hips, when you twist, bend, lift or stretch. You don't need to go spending 30 - 40 minutes specifically doing abdominal exercises. Especially crunching for long periods.

The following moves will target rectus abdominus and your obliques particularly. These run in a cross-hatch pattern down the side of your torso. Shamelessly indicating this in the image to the left. Stop staring, get to work!

Focus on gentle anchoring your belly button back towards your spine - not sucking in like you'll turn blue in the face, just feeling a 'brace' so that you can sense your tummy muscles are switched on. This is your first step - THEN start moving.
Side Step: ensure your full foot steps up to the bench before you extend - this can be done without weights if you prefer - keep your hips aligned beneath your shoulders so they don't tilt to the side.

Side Plank: you can do this on your knees - think about raising your hips to the roof.

Twist: you can use a rolled up towel or elastic strap - only lean back as far as you can keep your abs braced and DO NOT HUNCH YOUR BACK!

Side Step and Lateral Extension

Side Plank

Twist