Space mines (the things that go boom)

Use missiles, unguided, without propulsion. You can customise their warheads (sand, explosive, nuke, tachyon, ion, whatever) and add a sensor (proximity, magnetic, densitometer, NAS) to connect to the detonator. Since it's set in space, you might want to set the proximity sensors to practically adjacent to the ship.
Give them a Stealth coating to give your Sensors operator a -2 or -4 DM to spot them, and a Very Difficult Pilot check or two to navigate the field.
You're going to need some kind of propulsion on them for terminal guidance, or hitting a mine will be hitting a needle in a haystack. This also implies passive sensors, and active sensors for targeting on this terminal guidance - so the passive sensors alert the mine to the proximity of a ship, and the active sensors ensure it goes in the right direction to hit it. Both of these will have to be capable of detection at a distance, or ships will be able to get through the minefield at very high velocities, before the mines can move into their paths. Maybe this is ok, since the purpose of the mines might be to prevent ships from slowing down to get into orbit around a particular planet, for example, to refuel.

Static mines just sitting somewhere in space are kind of a non-starter, so let's try a "most favourable" scenario - such as a tail chase. Suppose you are being chased by a battleship. A battleship might be a 100m diameter circle of frontage; other ships will be smaller. If the ship is heading straight after you and taking no evasive action, all you need to do is drop a mine and it will hit. If the pursuer knows you have mines, however, it will devote a part of its delta-v to evasion. Assuming it only devotes 1G to evasion, 1G in a random direction within the 20 minute per turn time frame means the pursuing ship's actual location could be anywhere in a 7200km radius from the predicted location, if we assume it takes one turn for the pursuer to get to the pursued vessels previous location. So trying to move a .1km diameter ship through a 1600000000km2 area, without being hit by a mine seems possible, for any reasonable number of mines the pursued ship might carry. In fact, I think the pursuer would not choose to use 1 G for evasion, but substantially less, since it does seem to be overkill.

But put some rockets and terminal guidance on your mines to activate when the enemy is near, and the game changes completely. Even if the mine only has fuel to accelerate at 10Gs for 2 minutes, it can suddenly cover that 7200km radius.

Mines would effectively be missiles sitting in position waiting to be activated either remotely or by passing ships. More likely the later, since light speed lag would make it hard to activate them remotely at the right time.

They could be a cost effective way to deny enemy fleets access to volumes around key locations, such as gas giants. Defensively, clouds of deactivated stealth missiles could lie in wait, and be activated as mines when the enemy enters the system, to attack ships that try to refuel. Or minelayers could jump into the Oort cloud of a enemy system of logistical importance, but without the infrastructure to detect them, and put clouds of mines on slow ballistic trajectories allowing them to be captured into orbit around the system's gas giants. This could be accomplished deep within enemy territory .