Yes, you can create a drop-down field. I've created one for you on the demo now using your example. It's active on that same page.
If you have not been following the general Custom Fields thread, then some of what I've mentioned or am about to mention may be confusing, as well as why there are so many fields on the demo. Custom Fields were implemented as discussed in the spec here:
https://www.vaultwiki.org/issues/5523/#note32838
The page linked in my previous post is a test dump for every kind of custom field that needed more testing and to provide a bunch of examples in one location. We just made them with random names/text.
For clarification, the only "Infobox", per this request, is the box on the left (custom fields displayed using a XF2 widget). It's the standard width of a widget, just using the Sidenav widget position, which is a standard position in XF2; many admins don't use it at the same time as the Sidebar position, because as you've noted, it takes too much away from the main content. In production, you would likely choose either Sidenav or Sidebar for all your widgets, and not a mixture.
The style of the "Mood" is the standard style for XF2 custom fields that have the "color" field type.
The styling of the custom fields on the Details tab is mostly standard, with the exception of the date field. We updated the field to support times and also dates prior to 1900, so it is non-standard.
The style of the custom fields on their own tab (example "CustomTab" with URL-action "crazytabs") shows the field title as the tab name/header, with the content as the tab body. The example uses the "Wiki page chooser" type to embed a full page here (but that type also supports rendering as a link).
The examples "Simply Put" and "P.S." are testing "Value display HTML" which contains custom HTML but is a standard feature of XF2 custom fields.
The styling for all other custom field locations used as of this posting is the standard styling for custom fields in XF2, but the padding/widths of fields in the Infobox widget were tweaked slightly so that the fields would fit inside the widget container.