Nothing says the orbit of your planet has to be as close to circular as the Earth's. All orbits are elliptical, with the sun at one of the two foci. (Even a circle is an ellipse, it just has both foci in the same space).
The eccentricity of the ellipse is a measure between 0 and 1 of how stretched out it is; if it is very small the orbit is nearly circular. Earth's eccentricity is 0.02.
Regardless of any axial tilt; if the orbit of your planet is stretched enough, it will be close to its sun for part of the year (closest at perihelion) and far from its sun for part of the year (furthest at aphelion), and these can correspond to a hot summer and cold winter, respectively. On Earth, this is a variation of just 3%, but it can be made longer.
No elliptical orbit is off limits; Haley's comet is in orbit around the Sun, and it's distance varies from 88 million km to 5.2 Billion kilometers; i.e. inside the orbit of Venus (108 million km) to outside the orbit of Neptune (4.5 billion km); with corresponding super-heating and deep freeze.
So just a very little bit more eccentricity (stretching) of your orbit can do this, it doesn't have to be extreme. I don't have the formula for computing the solar energy reaching the planet (should follow a square law I think) or for translating that into average temperatures; but that is where I would take the research next. Perhaps other readers know this off the top of their head. I think this is the least exotic method of increasing the extremes between summer and winter, just make the planet about 10% further away from its sun in winter than it is in summer.