I second visiting Stirling. You could do Stirling Castle in the morning and then the Wallace Monument in the afternoon, after lunch. Its only a short drive from Glasgow, along the M80. The town itself is no great shakes, (OK for a lunch), but these attractions are great history:
www.stirlingcastle.scot/
www.nationalwallacemonument.com/
Then, returning to Glasgow late afternoon, have a night out. Perhaps - in the City Center - Le Chardon D'or for dinner, followed by drinks in the upstairs bar in the Blythswood Square Hotel around the corner. (The suggested restaurant is expensive by Glasgow standards, but not by Edinburgh or London Standards).
www.brianmaule.com/
www.kimptonblythswoodsquare.com/
Or perhaps, visit the lanes (around Byres Road) in the West End or Merchant City (east of City Center) for dinner and/or drinks. The former is reachable by tube (get off at Hillhead - its just around the corner), the latter by walking from City Center.
www.visitwestend.com/discover/the-lanes/
www.merchantsquareglasgow.com/
These links are to epicenters of nightlife, there is also much around about them. Walk about and explore.
Dinner and drinks in the West End may be preceded by a Walk around the beautiful Kelvingrove Park and Glasgow University. Bustling Great Western Road is also worthwhile, in terms of bars and restaurants. The Botanic Gardens are at the junction of Grt Western Rd and Byres Road.
Also in Glasgow, perhaps check out the following quirky / unique attractions:
sharmanka.com/Home/Welcome.html
www.nts.org.uk/visit/places/the-tenement-house
I would not bother with any of the "big" museum attractions in Glasgow, (eg Kelvingrove Art Gallery), they are grossly overrated imo - but then maybe they would be OK for a first time visitor (I am a cynic who has seen them a million times lol).
I would recommend checking out the Scotia Bar in Glasgow (down near the Clyde). Its a folk music bar, pretty rustic. Its always an experience going in. Extremely friendly, you cannot help but meet people (even if you do not want to lol). Last time i was there, for example, we met a Brazilian woman and her Irish husband. Previously it was a bunch of Americans. Its impossible not to meet people, drinking cheek by jowl - its so friendly. Bustling on weekend evenings, its great for a sing song and a chat. Its got a rough and ready pool hall upstairs, but the bar is best for visitors. Its sister bar, the Clutha Vaults, is nearby, but has never been the same since the Police Helicopter crash landed on top of it a few years ago.
www.belhavenpubs.co.uk/pubs/lanarkshire/scotia-bar/
If it was me, Id be in the Scotia every time, rather than the fancy bars of the west end of merchant city lol.
The other are right that the Highlands is a big ask in such a short time - really its the poor roads which are so limiting, especially if you get stuck behind a bus or other slowcoach.
However, beautiful Loch Lomond is only a shortish drive from Glasgow. If in decent health, you could climb Ben Lomond for some excellent highland-esque views (do it!). Or take a cruise on the Loch. Or visit the village of Luss, for a walk about and lunch or dinner.
Luss also has a "fairy trail" for families, but I believe you don't need to pay if you are just wanting to walk the route and not "do the trail" as such. The area is genuinely beautiful.
www.walkhighlands.co.uk/lochlomond/ben-lomond.shtml
www.cruiselochlomond.co.uk/
www.tripadvisor.co.uk/Attraction_Review-g551943-d13925530-Reviews-Loch_Lomond_Faerie_Trail-Luss_Loch_Lomond_and_The_Trossachs_National_Park_Scotla.html
en.wikipedia.org/wiki/Luss
Glasgow is a great base for a visit - I hope you this helps and you have a brilliant time. By all means ask any Qs etc.