Aye laddie, wan o' Jock Tamson's bairns.
May can be a good month in Scotland but the weather is never really predictable, suffice to say, drier in the east, wetter in the west. Always expect bad weather in the north west. The place is forty shades of green, covered in mountains, forests, lochs and rivers and nothing much between it and the North Atlantic.
Scotland is a small place so lots of stuff is pretty near other stuff. Stirling is well placed, plenty of historical odds and ends and not far from some of the best scenic bits like the Trossachs, Loch Lomond, etc. Ben Lomond is a relatively easy hike, a 4000 footer but really just a long walk. Have a look at:
Culture is Edinburgh and Glasgow, museums, galleries, concert halls and all that. Don't drive into Edinburgh, get a bus or train into the city. Waverly is the central station and is smack dab in the city centre at the foot of the castle rock. The two cities are entirely different in character, all the glitzy shopping is in Glasgow.
Wildlife is seasonal (no, get away!), the seabirds will be arriving to nest on cliffs around May, seeing that can be pretty spectacular. Dolphin watching in the Moray Firth and whale watching off the west coast - check on internet for these. May is a bit too early for basking sharks, more high summer for them. Rothiemurchus forest is good and the ospreys should be kicking around at Loch Garten and Loch of Lowes. Google the names, get loads of info'. Have a look at:
Could go on and on but won't. My opinion, remember it is only that; is if short of time, then Stirling base for first half of trip and Isle of Skye (pray for good weather) for second half. The drive up from Stirling to Skye can take in loads of things if planned carefully. Pack the wellies and waterproofs. Good luck!