Scratch Glasgow School of Art. Had second catastrophic fire in 2018 and will be closed for ages. Scaffolding blocks view of even the exterior.
Good places to learn about and/or admire Mackintosh's work (information gathered in 2019):
The Lighthouse, 11 Mitchell Lane, N of Argyle St; Center for Design and Architecture: Mackintosh Interpretation Center with models, etc. Free. Mon-Sat 1030–1700, Sun1200-1700. Subway: St. Enoch or Buchanan St; follow street signs to The Lighthouse. I think this is where you sign up for the Mackintosh walking tour.
Hunterian Art Gallery, 82 Hillhead St, across from Hunterian Museum. Good art. Free except for adjacent reconstructed home of C R Mackintosh (£3); tours Tue-Fri at 1000, 1030, 1100, 1130, 1200 and 1230. First-come, first served; no reservations. Self-guided visits to Mackintosh House studio-drawing room Tue-Fri from 1300 and all day Sat-Sun; 12 people at a time. Museum Tue-Sat 1000–1700, Sun 1100–1600. Last admission 45 min. earlier. Reading room: 1930s Art Nouveau. One of my top suggestions.
Kelvingrove Art Gallery and Museum, Argyle St (walkable from Hunterian): Art and natural history, focused on Scotland. Excellent and free. Enough Mackintosh work to justify visit. Mon-Thu & Sat 1000-1700, Fri and Sun 1100-1700. Donation ₤5.
House for an Art Lover (2-for-1 deal; ask if you need info), near NE corner Bellahouston Park, off Dumbreck and Paisley Roads, S of Clyde: Built 1996 from 1901 Mackintosh competition design. RR 5 min. from Glasgow Central to Dumbreck, then 10-min. walk; a bit hard to find. Excellent audio guide. M–W 1000–1600, Th–Sun 1000–1300. ₤3. A top recommendation.
Glasgow Art Club, 185 Bath Street, between Dalhousie St and Rose St: Mackintosh input on decorative elements. Gallery open to the public. Mon - Tue 1100 - 1800, Wed 1100 – 2300, Thu – Sat 1100 – 2400. (Walking tour goes inside.)
Willow Tea Room, 97 Buchanan St, is copy of Mackintosh design. (Walking tour passes by.) As I recall, the tea room is upstairs so you can't see much from the street. Sort of on the way to the Glasgow Art Club.
Daily Record Building, 20-26 Renfield Lane, between Hope St and Renfield St, very near river. Exterior only. Sight lines not great. Lower priority. (Walking tour passes by.)
Queen’s Cross Church (aka The Mackintosh Church), 870 Garscube Road, near Maryhill Rd: Mon-Fri 1000-1600 during summer. Bus: No 60/60A & 61 from Hope Street at Central Station. Subway: Alight at St George’s Cross then 15-min. walk through Cromwell Street into Maryhill Road. ₤2.50 senior entry. [I didn't have time for this one.]
Ruchill Church Hall, 15/17 Shakespeare St, E of Maryhill Rd and well N of Botanic Gardens: Exterior viewing only. Minor Mackintosh work, not very distinctive (based on pictures). Beside MacDonald's opposite Maryhill Shpg Ctr on Maryhill Rd (bus stops at both). Behind the Church is the Forth and Clyde canal with a bridge across to Ruchill. [I didn't have time for this one.]
Scotland St School Museum, 225 Scotland St, S of the Clyde: Mackintosh, but not very distinctively so. https://www.glasgowlife.org.uk/museums/venues/scotland-street-school-museum. Free. Tu-Th and Sat 10-17, Fri and Sun 11-17. Closed Mon. Opposite Shields Road Subway. First Bus 89 and 90. Skippable unless you're interested in the history of the school, which is the main focus.
Outside Glasgow but Helensburgh may be on your way west:
Hill House, CRM’s residential masterpiece (2-for-1 deal). RR from Glasgow Queen Street to Helensburgh Upper, walk up Sinclair Street (1.5 miles), turn right onto Kennedy Dr. and right onto Upper Colquhoun St. Top recommendation
Helensburgh Club, 38-40 Sinclair Street.Helensburgh Club, 38-40 Sinclair Street, Helensburgh. Tue-Fri 1100-1600. ₤3. [Closed when I visited on a Monday.]