Another thought, visit Stonehenge. I think you will find that the X5 (if not, the 5 does) stops at Larkhill Garrison Medical Centre. It is then an easy walk down to Stonehenge
https://binged.it/2Hcaz03. If you get a copy of the large-scale Stonehenge (and Avebury) map
https://retail.historicenglandservices.org.uk/stonehenge-and-avebury-1-10000-map.html, also available from Salisbury Museum or Stonehenge visitor centre shop, you will find there is far more to see than might be obvious if you take the expensive sightseeing bus from outside Salisbury station. I normally do the trip from Salisbury, but you can do it from Pewsey, or as stop en-route from one to t'other.
Only the X5 from Go-Ahead in the Salisbury area is covered on the ticket - and it's covered on all 3 (East, West and Global) and will be in the international one too.
The nearest the X5 gets to Stonehenge is the Amesbury - Pewsey Road (goes past Woodhenge if you'll settle for that). Approx 3 km walk to the stones, but I suspect you would not be able to gain access from that side (can anyone confirm?) ... and you would have several more kms to walk to get around.
Other Go-Ahead services in the area that would facilitate visits to Stonehenge (get you closer to the centre)
* The Stonehenge Tour, which goes to the visitor centre
* The X4 that goes to Larkhill Garrison for a walk (circa 1.5 km)
* The 2 (Salisbury to Devizes) for a walk (circa 2.5 km, not easy road)
in addition
* The Activ8 (Salisbury to Andover) offers additional journeys between Amesbury and Salisbury
* The 201 (Tourist Coaches and StageCoach) operates a very limited service via West Amesbury.
The X5 timetable includes some journeys (early morning and end of the day) which do call at Larkhill Garrison, but the timetable says "operates as X4". By the letter of the ticket description, you would not be allowed to use these services on the Great West Way ticket as it's specific to X5 services. In any case, the early morning ones are before 09:30 so would have only been valid on the International ticket.
The stated marketing aim of the Great West Way is to encourage independent travellers from overseas to visit the many wonderful tourist attractions and countryside we have along the corridors from London to the River Severn - taking in a wide swathe of Berkshire and Wiltshire as well as just what's visible from the A4. And as such a marketing / business decision has been taken not to include services that make access to the "Honey Pots" easy and relatively cheap. Hence the exclusion of services close to Stonehenge.
There are some other excellent things to see from Salisbury (easily reached by train on the ticket) and Amesbury.
Old Sarum -
https://www.english-heritage.org.uk/visit/places/old-sarum/Boscombe Down Aviation Collection -
http://www.boscombedownaviationcollection.co.ukAmesbury History Centre -
https://www.visitwiltshire.co.uk/things-to-do/amesbury-history-centre-p1536253And Amesbury is an interesting small town to look around in its own right.
Using your Explorer ticket? Be sure to board the X5 and not the 7, X4, 201 or Activ8 services from Salisbury to take in these places.
Go-Ahead South Coast timetables for services near to Stonehenge mentioned at:
https://assets.goaheadbus.com/media/timetable_pdf/X4_X5_Sept_18.pdf .
https://assets.goaheadbus.com/media/timetable_pdf/reds_preview_web_tt_Activ8_1118_PRESS.pdfhttps://www.salisburyreds.co.uk/timetables/park-ride-7-31-may#timetablehttps://assets.goaheadbus.com/media/timetable_pdf/2__Sept_18.pdfhttp://www.thestonehengetour.info/timetable