It’s actually quite easy if you look at the documentation of some of the SPSiteCollection.Add() methods which take an int as a compabilityLevel parameter. Passing in 14 in this parameter will ensure you get the 2010 look and feel. For reference the SPSite.SelfServiceCreateSite() method has similar overloads.
SPWebApplication webApp = new SPSite(http://host).WebApplication; SPSiteCollection siteCollections = webApp.Sites; uint lcid = 1033; int compatLevel = 14; string webTemplate = "STS#0"; SPSite newSiteCollection = siteCollections.Add("sites/test", "Title", "Description", 1033, compatLevel, webTemplate, "DOMAIN\\User", "Name", "Email_Address", "DOMAIN\\User", "Name", "Email_Address");
If you want to do it I PowerShell you can use the following command:
New-SPSite -Url http://host/sites/test -OwnerAlias "DOMAIN\User" -Template "STS#0" -CompatibilityLevel 14
And via Central Admin: