The easiest thing that I do, is take a handful of green beans from the can we opened for dinner, lay them out on a piece of foil & freeze until solid. Then gather up the beans into a zip-loc bag for easy storage. I remove a few and let thaw slightly before dunking in the tanks.
I'm super OCD, I split open the beans & remove the seeds/peas.
I also just slice up a washed zucchini into 1/4" slices, freeze on a piece of foil and dump into a zip bag. I thaw slightly, stick with a fork and toss it into the tank. My snails (briggs, ramshorn & mts) can finish off a piece in 1 1/2 days (I wouldn't leave it in longer than that).