Why can’t I stack the Experience Scrolls

Left-clicking on the ground is the most reliable way to deselect all targets.

ESC deselects any selected targets, but if nothing is selected you will bring up your menu (can be annoying mid-combat).

There is also a key binding to target the nearest ally, which will select yourself if no other allies are around. This is useful if you are having trouble finding a patch of ground to click on. This key is not assigned by default, so look for "Nearest Ally" under Targeting in Options->Controls.

You can also select the next closest target using TAB, in case you do actually want to direct your spells towards a different enemy then the one you current have selected. Then again, a left-click on the target works more reliably if multiple targets are in view.

There is also a Guild Wars 2 printable keyboard layout to help you visualize the default key bindings.

Tonics now have two additional uses (at least, during the Halloween event);

1) They can be consumed to be turned into a mob, which has combat abilities in the Costume Brawls, which also have their own achievement track.

2) Mystery Tonics are part of a recipe for the Mystic Forge, which allows you to create Mad King's Chests (no BL Key required!). Six Mystery Tonics, a piece of Candy Corn, and two different boosters (Killstreak, Karma, Regen, Magic Find, etc.) will create the Mad King's Chest. These chests have a chance to drop rare Halloween weapon skins.

Note: Now that the Halloween event is over, the Mad King's Chests are no longer available for crafting. They may or may not be implemented again next year. Mystery Tonics still turn you into various creatures that participate in costume brawls.