Any trips I look at us doing, I will write up a basic itinerary here. I basically look at the parks websites for opening times, search for flight information (like early flights out and late ones back) car hire and hotels.
Once I have the basics I will start to dig deeper in to planning each trip.

Germany - Hansa & Heide Park, September 2011
Friday 2nd September
Fly from Manchester to Hamburg. Drive to Hansa Park to spend afternoon. Drive to Heide to stop at their on site hotel, if late, stop off in Hamburg for dinner.
Saturday 3rd September
Heide park all day.
Sunday 4th September
Heide park till 2pm then leave for airport. Fly Hamburg to Manchester.
Cost approx £300 per person (based on 5 people sharing)

Italy - Gardaland & Mirabilandia - planned for June/July 2012
Thursday 7th July
Fly from Manchester to Venice. Drive from airport to Mirabilandia. Spend night in local hotel.
Friday 8th July
Spend most of day at Mirabilandia. Leave park around 2pm and head to onsite hotel at Gardaland.
Saturday 9th July
All day at Gardaland and 2nd night in hotel
Sunday 10th July
Spend morning at Gardaland. Leave park 2pm for flight home. Fly Venice to Manchester.
Cost approx £420 per person (based on 5 people sharing)

Spain - Parque de Atracciones de Madrid & Parque Warner Madrid
Friday 2nd September
Fly from Liverpool to Madrid. Drive from Airport to hotel, check-in then get metro over to Parque de Atracciones de Madrid. Back to hotel at park close.
Saturday 3rd September
All day at PWM. Night at hotel
Sunday 4th September
Check out of hotel and most of day spent at PWM. Leave park around 5pm for aiport. Flight Madrid - Liverpool
Cost approx £225 per person (based on 5 people sharing)