I guess that depends on what you mean by fiddle. If you are talking about playing around with the drive system, probably not a good idea. A few HSD systems have been modified for more battery power / custom plug in but I would not put that into the "fiddle" category.

If you are talking about accessorizing, then its about the same as any car.

Two climate control systems, one inside and the other at the tailpipe.

2007 Camry Hybrid (in service June 2006)
FE: 6.77L/100km (34.8 mpg) over 37750km (23450mi)
75% city - Ave trip 20 min - 2 Winters