Halo Fountain and Incubation Druid do not have summoning sickness.
Incubation Druid has at least one +1/+1 counter on it.
Steps
Activate Karn's first loyalty ability by putting a loyalty counter on it, causing Halo Fountain to become an artifact creature until end of turn.
Activate Incubation Druid's first ability by tapping it, adding ({W} magic symbol) ({W} magic symbol) ({W} magic symbol) .
Activate Halo Fountain's second ability by paying ({W} magic symbol) ({W} magic symbol) , tapping it and untapping it and Incubation Druid, causing you to draw a card.
Repeat from step 2.
Results
Infinite card draw.
Infinite draw triggers.
Near-infinite colored mana.
Near-infinite creature tokens.
Near-infinite untap of creatures you control.
Near-infinite mana creatures you control can produce.