I lived there for a few years, though I wasn't doing much biking back then. What trails etc there are, are very short and often quite difficult to connect to make a ride of any significant length TBH. There is a nice ride to the Bothy at the top of Glen Forsa, Tomsleibhe, which would make a nice overnighter. There is also part of a Dakota from '44 that crashed on Ben Talaidh that you can walk to from the bothy, its basically up the burn line heading toward the saddle, around the 400m contour line. There is a bit of a ride out to a spit of land and small island group called Cuan Mor from Glengorm castle, there is an old Dun out that way too. At the headland at NM420572 there is some great rock pools that the kids would love(assuming they aren't too old for all that nonsense!), good views over the Ardnamurchan and Ben Hiant etc.
For beaches the best one is at Uisken, just south of Bunessan, most folk go to Calgary but it is often crowded 'cos of this, and Uisken is a nicer beach anyway. Great views all the way over to Colonsay and the Paps of jura beyond.

Looking over the rock pools, if the tide wasn't in that is!

Same headland looking toward Dun Ara and the mainland, Ben Hiant and Kilchoan.

Uisken beach, the wavey bumps in the background are the Paps of Jura

Allt Forsa, looking to Ben Talaidh. The plane wreck is almost at the very top of the burn that runs down from the saddle in the centre of the 2 peaks. Bothy is under the right hand ridgeline, as it were.
As you can see the island is pretty vaguely exciting, and we have barely touched upon Iona etc! If the kids like playing on beaches act they will love it, plus there are whale/dolphin watching trips etc and if your lucky the Mobile cinema may even be in town, Tobermory and\or Craignure. What's not to like?