You rang?
To be fair there isn't a shedload of stuff to do around here lol
There's Outburst (
http://www.outburstsalisbury.co.uk/clownabout.php), which amongst other things has a huge indoors soft playarea for kiddies. You could easily kill an hour or two with a little'un there.
Town is a short walk/drive away which has lots of restuarants if your kid works up an appetite, Outburst does have food on offer, but it's not a massive range.
There's Finkleydown Farm just up the road in Andover (
http://www.finkleydownfarm.co.uk/) which has farm animals and play areas etc Adults are 7GBP (pound sign doesn't work lol) kids under 2 go free, that could kill a couple of hours easy.
There's a treasure hunt apparently, (
http://www.dayoutwiththekids.co.uk/family-fun/Huntfun_Salisbury_Treasure_Hunt/2027), but might be a bit hard with an 18month old in tow?
Splash of Colour might be fun if your kid is arts and crafts orientated (
http://www.splashofcolourcafe.co.uk/index.php)? Cannot reccommend it highly enough, really good fun, if you're into that sort of thing.
There's Wilton House, which is just down the road from me, is a museum, has a nature walk, and lots of kids play areas, can easily kill a couple of hours here too.
Just a couple of ideas to help you out? Other than these things, Salisbury is a bit limited on kiddy stuff!