book open jaw and such - use Multicities option

You can do that using multicities option on virtually any site, airlines own sites and Expedia / Travelocity sites.

just manually key in your itinerary and it would come back with fare. Make sure you key in the same date on your first cities upto the destination if you need connection.

In the above case, it is the same price for me to return from LHR or from CDG so I picked CDG. We have no idea yet where will we eventually go or what to do as this is too far out in Sept. We have 2 trips before that (late May to HNL/OGG, August to PDX and Northwest). However the options are almost endless. One thing for sure is we would not waste time on Museums on Summer trips to Europe - those can be done in Winter trips.