I used to live near Dunstaffnage Castle, which is close to Oban in Argyll and have visited it many times.

The castle is maintained by Historic Environment Scotland. Access to the grounds and the chapel in the woods are free. There is a charge for access into the castle, although if you are a member of historic environment Scotland, then its free. There is parking, and from there it is a few mins walk to the castle, along a pretty good road.
The castle is on the tip of a headland, which has commanding views over the Firth of Lorn. The views up towards Connel and the mountains behind (including Ben Cruachan) are quite spectacular.

Although the castle is a ruin, you can safely get up to the top of the walls, there is also one remaining standing building inside which you can get access to. There is also the obligatory castle well, and remains of a number of other rooms/features.

The views from the walls are fairly impressive - in the shot below you can see the walls on the stable block that used to sit outside the castle.

The castle is based on an ancient volcano tor which gives it an impregnable base - and the walls are very thick - for the obvious defensive reasons.

Its got lots for kids to run around and climb on so is great for a family visit.

And the grounds are popular with dog walkers and people out for a general stroll.

All in all, we think its well worth the visit - it has lots of history, and stunning views all round.
Links:
If you want to support important buildings, history and heritage in Scotland and the UK, then please consider the following:
For a reasonable annual membership price, both provide access to numerous historic properties across Scotland which are great for days out. Once you've been a member for more than a year you also got free access to other properties across England, Wales and Northern Ireland (including half price entry during your first year) - so its well worth the investment, plus its all for a good cause.