For lettuces I always have green and red leaf lettuce on hand, which all the pigs enjoy. Star-Lord and Rocket don't care for the pieces with thicker stalk close to the base. They'll eat the leafy parts and leave the rest. I also rotate through escarole, endive, butterhead and a few other lettuces depending on what's in season and what's decently priced. All 4 pigs love radicchio, so I usually put in a few leaves of that with each meal. They get 1/8 to 1/4 of a bell pepper a day split up between the two meals. They also like the mini sweet peppers, and I'll core the center out and rinse the seeds and either just give them the whole pepper or slice them into rings. All 4 pigs love bell peppers. Gus likes to steal peppers from the baby, mainly because he's not quick enough to eat big chunks, so Woodrow gets an Oxbow vitamin C tablet, and if I feel he didn't get enough, I'll bring him out for lap time and let him have a few extra pieces. These are the staples of all my piggy salad meals.
For the extras, I rotate through either cherry or grape tomatoes, halved, one per pig, a slice of cucumber, or carrot chips/baby carrots and cilantro. I try not to feed the same "extra" twice in one day, and with carrots and cucumbers I try and stick to just one or the other every couple of days.
For fruits we did watermelon in the summer, and maybe once or twice a month I'll do a little piece of apple. They all love blueberries, but I keep those to a little treat.
As for likes and dislikes...Gus will eat pretty much anything but cilantro is a last resort. If it's there when he's gone through everything else, he'll eat it. They all like peppers and carrots, the baby especially likes the carrots. Woodrow is hit or miss with the tomatoes. I haven't found anything Star-Lord and Rocket don't really like save for the thicker part of the lettuce stalks.
Six of the seven love canned pumpkin (only the 100% organic pumpkin with no additives). The seventh won't touch it.
All love carrots, any kind of lettuce, celery leaves, cilantro.
I give them Oxbow's treats made with timothy hay and peppermint when I can find it, and they absolutely fight over those, even moreso than other treats made with the same base. I haven't tried actual peppermint plants.
Can guinea pigs eat fresh asparagus?