You can log IFR when ever you like as long as you obey the IFR rules. Ie above MSA etc flight quadrangal rules etc.
You can depart VFR climb above MSA fly your route in perfect VMC then decend and recover VFR the portion of the flight when your obeying IFR can be logged.
And all time at night out side controlled airspace can be logged as IFR as well. The stuff inside can be logged as well as long as your not under a SVFR clearance
If you depart on a IFR flight plan and return without canceling the plan yes you can.
If you book out VFR to do some handling you can log some of it. as per the the first option.
But don't for gawds sake don't get confused with "sole reference to Instruments" that can only be logged when your under the shields. Or actually flying in IMC. The defintions all changed with JAR and the old system is quite a bit different. I am expecting some inbound from people trained under the old CAA rules because they were different with different requirments for license issue. The sole reference to instrucments isn't really relavent any more unless you want to teach applied instruments as an instructor. For ATPL issue i think it asks for 60 Instrument hours which you will cover in your CPl and IR courses and nothing actually defines what instrument hours are if its sole reference or IFR.
MJ