Foodstuffs you can't offer to Krishna, and why:

""If one offers Me with devotion a leaf, a flower, fruit or water, I will accept it."   (Bhagavad Gita 9.26) 

Everything in this material world belongs to  one of the three "Modes of Nature", Satva (Goodness), Rajas (Passion) or Tamas (Ignorance). to cultivate our spiritual life, we should only consume those foodstuffs in the mode of goodness. It is not just bhakti yoga which teaches this, but other types of yoga as well, as many yoga books and websites show.

Avoid:
1: Meat, fish, eggs, alcohol, drugs and stimulants: These are in the Mode of Ignorance, as the Vedas teach us. Anything to do with killing, cruelty, exploitation will only serve to foster in us that state of mind and further cover up our radiant real inner selves (the "jiva" or soul). We will suffer in accordance with the amount of suffering we inflict upon other beings- this is the Law of Karma.We are currently in the age of Kali Yuga, where suffering, violence, quarrel and hypocrisy reign supreme: we need an antidote to this, not to perpetuate it. God is above the three modes which govern the material world (goodness, passion and ignorance): He is only pure Love.
2: Onions, garlic: These may perhaps be occasionally taken as medicine (although there are certain toxic as well as therapeutic compounds therein), but not eaten as a matter of course or offered to Krishna. They are in the Modes of Passion and Ignorance, and therefore engender in us a restless, impulsive, violent, overly passionate and sense-gratifying nature which is not conducive to the cultivation of a peaceful mind and a progressive spirtiual life as they detract from bhakti (our love for and devotion to God).
3: Mushrooms: "Mushrooms from the dark forest", says the Srimad Bhagavatam  (a Vedic scripture), should not be eaten. Because of where they naturally grow, on decaying matter and in dark places, they are not considered to be in the mode of goodness.
4: Carrots, red lentils (masoor dal), sea vegetables: Thousands of years ago in Satya Yuga (the "Golden Age"), when a brahmana sacrificed a cow, it did not die but was rejuvenated. But then one brahmana's wife, who was pregnant and had a craving for meat, took a piece of flesh from the cow before the mantras had been said to bring it back to life. When the mantras were said, the piece of meat also came back to life, so the woman threw it away. It landed in the sea, and where the flesh, bone and blood fell as it travelled, carrots, onions, garlic and red lentils grew. One of the foremost living exponents of bhakti yoga, Srila  Bhaktivedanta Narayana Maharaja, confirms the veracity of this,saying that these foods should definitely not be eaten. He does not mention seaweed directly in that particular conversation however. There is an argument which goes that carrots in those days were not the same plant that we call carrots now (they were black or purple, and orange carrots only appeared in Renaissance times), and one which states that it is only the dark red Indian carrots which are prohibited. Srila Bhaktivedanta Swami Maharaja ("Prabhupada"), revered founder of the International Society for Krishna Consciousness, permitted his disciples to eat carrots, but said that this was an allowance as Krishna Consciousness was relatively new to the West and there are very many rules in bhakti yoga. All I can say is:  do your best according to the time, place and circumstances you are in.
I do not include any of these prohibited foods in my recipes here, but please know that I am not perfect and do slip up from time to time... but please try this yogic diet; you will feel a difference in your consciousness.
You can find more details at: www.purebhakti.com (discourses section) and www.salagram.net