Often with some parks (especially first visits to them) it's worth taking time and just enjoying them.
Cred runs can also be fun don't get me wrong, but being able to just take it in is also a great benefit.
Looking at map might be worth just doing France, Germany, Holland and Belgium parks. Potentially as below:
1 - Travel to Paris
2 - Asterix
3 - Travel to Lyon
4 - Walibi Rhonde
5 - Travel to Europa/Rust
6/7 - Europa
8 - Travel to Cologne
9/10 - Phantasialand
11 - Travel to Hamburg
12 - Hansa Park
13 - Heide Park
14 - Travel to Amsterdam
15 - Amsterdam
16 - Walibi Holland
17 - Travel to Ghent
18 - Plopsaland
19 - Travel to Brussels
20 - Walibi Belgium
21 - Home via Eurostar
As mentioned you'll need to check on how easy some places are via public transport (Hansa & Phantasialand definitely are from Cologne & Hamburg). Though if inter-railing it most of the costs might be covered. Also if the journeys are a bit shorter have more time to relax or look round the cities.
Only given Heide one day because it really isn't worth more.