My bunnies normally get veggies once a day. In the summer more than in winter, because we have a big garden and tons of veggies. So when I have a huge selection of veggies to give them I only feed pellets as treats, in the winter time they get them every day (but still only a small amount).
As far as the veggies go we feed a different mixture of following veggies: parsley, basil, mint (different kinds), oregano, rocket/arugula, dandelion, different salads and lettuces, endive, radicchio, cucumbers, bell peppers, spinach, kale, cabbage, carrot tops, swiss chard, bok choy, broccoli, raspberry leaves and strawberry leaves.
As treats they get a few times a week small pieces of fruit/high carb veggie depending on what is in season: carrot, nectarine, peach, plum, apple, strawberries, raspberries, cherry tomato, banana, raisins, cranberries, dry apricot etc
They also love to nibble on dandelion roots or certain woods: willow branches, apple tree branches, birch etc.
Portion size of the veggies depends on the size of your bun and also on the fact if he is getting pellets or not. I kind of learned with time how much to give them by observing their poop size and consistency.I would say they each get: 1 big handful of fresh veggies in winter and 2 handfuls in the summer. Oh and my bunnies all weight around 1,5 kg.
This blog post might help you with portion size:
https://bunnyapproved.com/bunnys-pellet-free-diet/
https://bunnyapproved.com/no-more-pellets-an-introduction/