The Army Aviation Museum at Middle Wallop (EGVP) is well worth a visit, and traces army and RAF activity from WW1/2 and Korea to recent campaigns. It is well presented and seldom crowded. You can land at Middle Wallop, subject to having an appropriate insurance clause (known as the MoD waiver), but it is expensive for a grass airfield and bureaucratic.
Best option is to land at Thruxton (EGHO) which has a good restaurant and from where they will get you a reasonably inexpensive taxi for the 5 mile journey.
Jujupilote wrote:
Does anyone know a good submarine museum (with one to walk around inside), in the UK or Germany ? Close from a friendly airfield if possible
Slightly further afield (but still in Europe!), Tallinn, Estonia, has an excellent maritime museum containing a British built pre-war submarine Lembit housed indoors in the former seaplane hangar. It’s a lovely town, too, and affordable. Except the airport, which makes up for the inexpensive hotels and restaurants with a 100 Euro fee. You can’t have it all, can you?
Jujupilote wrote:
Does anyone know a good submarine museum (with one to walk around inside), in the UK or Germany ? Close from a friendly airfield if possible
In Bremerhaven you can visit the "U-2540 “Wilhelm Bauer”,":http://u-boot-wilhelm-bauer.de/en/ an actual Type XXI U-Boat built late in WWII and later refurbished for the post-war German Navy.
Unfortunately Bremerhaven Airfield closed last year. The city is now serviced by the joint-military Nordholz-Airfield 35 km to the north (ETFM) or alternatively go to Bremen EDDW which is a GA-friendly airport 50 km to the south (beware of expensive Avgas though, best refuel in Leer EDWF which is the best VFR GA airfield for the surrounding 100 nm or so 
). Both Nordholz and Bremen have train stations connecting them with Bremerhaven.
Another option is the Muckleborough collection in North Norfolk.
The airstrip at Weybourne is a short walk away. Possibly unmanned, look out for radio modellers and sign in with your donation at the caravan (at least that was how it worked last time that I visited).
The very best submarine exhibit I have seen is the nuclear missile submarine at Cherbourg, N France. They cut out the middle reactor section but all the rest is there and you can walk along the missile tubes.
There is a lot of Navy stuff at Portsmouth (near Lee on Solent, UK) but not on the scale of the above.
Croatia is full of old military stuff. @Emir will know. Mali Losinj (LDLO) has amazing tunnels which are not (yet) locked up as they would be everywhere else. Brac (LDSB) has submarine pens but no subs
Prague has plenty too, in an aviation museum near Letnany LKLT. @Ultranomad will know… Some pics here. In fact there are many Czechs on EuroGA (apart from me).
As some may know, The French Musée des Blindés (Tank Museum) is located in Saumur in the Anjou area.
The Saumur airfield is LFOD. It has a long hard runway with no lighting. The aeroclub is friendly and can sell you AVGAS.
You will need a taxi to go to the museum. If some are interested to visit, I’ll try to join the group.
Does anyone know a good submarine museum (with one to walk around inside), in the UK or Germany ? Close from a friendly airfield if possible
The Jutland Museum at Thyboron, Denmark makes an interesting visit with many artifacts from the battle and other engagements in the N Sea, including the complete conning tower of a British WW1 submarine. It’s less than 10 mi from the delightful glider site at Lemvig by a direct and rustic train service. Lemvig (Grass, but excellent) is happy to accept visitors and there’s a nice hotel nearby. If you have family, nearby Billund has an airport and the Lego museum!
Peenemunde is indeed worth a visit. The site is good although it has been comprehensively stripped down by the invading forces in 1945. The museum is quite new but is rather bare. In 2013, they did have toilets there but virtually no edible refreshments – I was told this is intentional to keep it from becoming a high profile site.
Thanks for the trip suggestions.
May I also suggest this one: Peenemünde (WW2 rocket development site) has an airport at 1km distance.
For anyone interested in the history of tank warfare (from WWI until today) I strongly recommend the
Deutsches Panzermuseum in Munster
It started as a collection of various tank models to teach soldiers of the German Army but has since been opened to the general public and nowadays encompasses a large collection of armoured vehicles from all periods and many different nations.
The next GA airfield to it is a 900 m Gras strip in Hodenhagen EDVH, but it may be easier for foreigners to just fly to Hannover EDDV and use a train from there.
Note: Very important: Munster is not Münster! Umlauts matter! Münster is a university city in North-Rhine Westphalia while Munster (aka Munster (Örtze) is a small garrison town in the Luneburg heath, in Lower Saxony. Münster has an international airport but no tank museum AFAIK
