I don't get why everyone is saying to travel so far, particularly Skye. 2+ hours sat in a car passing by some of the best places in Scotland? Then sit in a queue home for 3 hours behind more visitors doing the same? Madness...
So:
Glencoe itself: do a mountain day, the Pap of Glencoe is a classic.
Picnic down Glen Etive - find somewhere to park, next to one of the multiple waterfalls. Brilliant in nice weather.
Day out on Lismore - take the wee ferry from Port Appin and either walk or hire a bike (on Lismore) to cycle to the community cafe, the deserted beaches and stunning views south over the Inner Hebrides. You get to see Castle Stalker on the way too.
Dunstaffanage and Ganavan Sands are lovely beaches - and you can combine with some Oban wandering and shopping. The LULU lounge at Perle Oban Hotel has great coffee and cake at less cost that Costa opposite ;-)
Another Island - Kerrera - is a fab day out on foot or bike. Just in the bay from Oban.
Mull - you can take the ferry and bus, and Tobermory is nice, but it is a long way. You will not likely get a car on the ferry, it will be fully booked.
Fort William - lots of shops, but the Nevis Range and cable car is well worth a visit on a good day.
Glenfinnan - just gets silly busy. Go beyond out to Mallaig and find the beaches...amazing.
I have enjoyed wandering around Eriska, and saw otters on both visits.
I also like Glen Creran, just turn after the bridge at Creagan and park where you can for a wander up to the Fairy Bridge.