What happens if everyone gets busy and doesn't sign a renewal?

Leases are full of words like "if... then". If the Tenant doesn't pay, they pay a penalty. If the Landlord doesn't give possession at the start of the Lease, the Tenant gets free rent until possession is given. But what happens IF the Tenant is happy with the space and the Landlord is happy with the Tenant and at Lease expiration the Tenant just remains in the space?

Make sure you are happy with what automatically happens at that time. Should the rent change? Should reimbursements change? Should one party be required to do some sort of improvement to the property?