Its ideal if these creatures do something when they enter the battlefield or when they die, and its ideal if they can send themselves to the graveyard, especially without spending mana, or at least not much mana
While this shouldnt be too hard for cards like Sita Varma, Masked Racer, Loots multitude of abilities could get difficult
Ibs ability to sacrifice two Mountains to generate two 1/1 Goblin Tokens also creates creatures in a pinch
All Secret Lair drops are available for a limited time, with a limited number of cards printed for the drop
Blue and green support each other incredibly well and make up for their respective differences
If you choose an artifact creature card or an enchantment creature card in your graveyard as the target, you could exile that card as the triggered ability is resolving, but if you do, you won't return it to your hand