I would explore the options in the Spey Valley on this map on which you can see Fort Augustus
Head south on the A82 and then via A86 head east to Newtonmore. Stay off the A9 and head north on the old road to Kingussie and Aviemore. You can click on any of the villages and visitor attractions on the map to see what might be of interest but the area around Cairngorm Mountain is very scenic with some lovely low( and high) level walks. Loch Morlich and Loch an Eilein are beautiful and the walk around Loch an Eilein amongst the beautiful pine trees is not to be missed.
You could make a round trip of it by returning to Fort Augustus going onto the A9 north of Carr Bridge ( the Old Pack horse bridge itself worth a visit) and taking the B851/ B862 not shown on the map link I gave you but a nice route nevertheless.
This would be a long day out with a lot of driving but with plenty of things to pack into it!
In May you will have a lot of evening light and its my favourite month up here in the Highlands because the trees and plants are finally bursting into life.
Head for Invergarry or Invermoriston and then take the road to Skye driving through Glen Shiel, you could detour to Glenelg to see the ancient brochs (even if you won't drive all the way take a short detour of a couple of miles to some picnic benches on the hill and a fantastic viewpoint back down the bay and over the Five Sisters Of Kintail). Stop at Eilean Donan Castle, visit the lovely village of Plockton, driving through the village of Duirinish on the way where Highland Cattle are often roaming in the village. You could then take the bridge over to Skye and just see how the time goes before heading back to Fort Augustus. To give you an idea of driving time from Fort Augustus to the Skye Bridge without stops (you will make loads though :-) ) is just over an hour.
From Fort Augustus (you may well have driven part of this route on the way up but without the time to detour or explore) head South towards Fort William, go for a ride on the Gondola at Nevis Range http://www.nevisrange.co.uk/gondola-info.asp drive up Glen Nevis and take the walk to Steall Falls if you like walking. Visit Neptunes Staircase at Banavie just outside Fort William, keep going along that road to Glenfinnan and the Glenfinnan monument as well as the Viaduct (also know as The Harry Potter Train Bridge). There's much more but they are a few ideas to get you started on a day out in that direction.