Open default browser app

app.SendIntent(null, null,
"android.intent.action.MAIN",
"android.intent.category.APP_BROWSER");

Open default music app

app.SendIntent(null, null, 
"android.intent.action.MAIN", 
"android.intent.category.APP_MUSIC");

Open default calculator app

app.SendIntent(null, null, 
"android.intent.action.MAIN", 
"android.intent.category.APP_CALCULATOR");

Open default contacts app

app.SendIntent(null, null, 
"android.intent.action.MAIN", 
"android.intent.category.APP_CONTACTS");

Open default email app

app.SendIntent(null, null,
"android.intent.action.MAIN", 
"android.intent.category.APP_EMAIL");

Open default maps app

app.SendIntent(null, null, 
“android.intent.action.MAIN”, 
“android.intent.category.APP_MAPS”);

As an alternative, you may wish to open Google Maps specifically - the code should really be on this separate page

app.SendIntent(
  "com.google.android.apps.maps",
  null,
  "android.intent.action.VIEW",
  "android.intent.category.DEFAULT",
  "geo:0,0?q=34.5678,123.4567 (Placename)"   // uri argument
);

Replace the coordinates and Placename in the uri argument with those for the desired location. (Optionally, you can omit Placename along with the parentheses.)

Open default gallery app

app.SendIntent(null, null, 
"android.intent.action.MAIN", 
"android.intent.category.APP_GALLERY");

Open default calendar app

app.SendIntent(null, null, 
"android.intent.action.MAIN", 
"android.intent.category.APP_CALENDAR");

Open default market app

app.SendIntent(null, null, 
"android.intent.action.MAIN", 
"android.intent.category.APP_MARKET");

Open default messaging app

app.SendIntent(null, null, 
"android.intent.action.MAIN", 
"android.intent.category.APP_MESSAGING");

Open bluetooth settings

app.SendIntent(null,null,
"android.settings.BLUETOOTH_SETTINGS" )

Open WiFi settings

app.SendIntent(null,null,
"android.settings.WIFI_SETTINGS" )

Other Settings

See https://developer.android.com/reference/android/provider/Settings
The Constants section contains Actions (use as 3rd parameter on SendIntent,
replacing ACTION_ by android.settings.)