Overheating 2009 Nissan Cube

There are a handful of parts to the cooling system.
Engine (coolant passages, head gasket)
Water pump
Radiator
Radiator cap
Radiator fan
overflow bottle
Hoses
heater core

There are a couple reasons for overheating. Insufficient coolant, insufficient coolant flow, inability to build pressure, and insufficient air flow across the radiator.

Firstly, make sure you fill up the system and bleed any air out of it. Air pockets can lead to overheating. Leave the heat on, fill it up, get it up to temp, let it cool down, check it, top it off. Some cooling systems have an air bleed port. Make sure the overflow is filled to the line, as this acts as a 1 way check valve and helps to bleed the system of air.

Secondly, coolant flow. Does the heat work? Does the upper radiator get hot when the engine is hot and temperature gauge are reading hot? Coolant flow issues can be one of a few things - bad thermostat (heat works, but top radiator hose is cold), plugged radiator or heater core, or a bad water pump (overheating and heat doesn't work well or at all)

Pressure - you mentioned it was leaking from the cap. There cannot be aby leaks or the system will not be able to build pressure. Being under pressure raises the boiling point (3* F for every PSI) and helps keep the system from boiling over. When the engine is hot, the upper radiator hose should be hard.

You mentioned the fan works, so airflow across the radiator should be good. You mentioned accelerating helped cool it, so I wonder if the fan is moving enough air. Overheating while stopped, but not while moving points toward the fan.

An easy way to know you blew the head gasket is if your exhaust smells like coolant, or you oil or coolant look like a milkshake. Another way is to pull the plugs to see if one is steam cleaned.