With the introduction of apps for SharePoint, many have speculated that sandbox solutions are dead/deprecated. This is accurate for solutions containing assemblies running on the Sandboxed Code Service (aka – SPUCHostService.exe). However, declarative solutions are very much still in play and widely used internally by SharePoint (ex: Web Templates and Design Manager). A declarative .wsp package (one containing no assemblies) is a powerful way to provision elements into the host web. So don’t be afraid to leverage the solutions gallery as long as your .wsp packages don’t contain code.
If your solution contains code and a farm solution isn’t an option (read: SharePoint Online), you might consider an approach I outlined in my recent post on App Approaches for Common SharePoint Customizations. In it, I discussed the use of app installed remote events to provision elements in the host web. This pattern is generally discouraged, as apps for SharePoint should uninstall cleanly (hence the reason for an app web). However, it is appropriate for internal apps (not marketplace apps) containing programming logic and host web dependencies. The app model should not be confused as a deployment mechanism, so if your solution doesn’t have code…consider the solution gallery.