I've been to a few foreign GPs in the last 7 years or so. Some are easy to do, some need a bit more planning.
Prague I have done a couple of times, easy access as it has it's own airport, food and drink are very cheap, hotels are plentiful, trams run from the city centre to within 5 minutes walk of the stadium. They also serve beer in the stadium that you can take to your seat unlike the Polish venues, I think a round of beers for 9 of us in the stadium was about £12!!
Bydgoszcz has an airport, but flights tend to sell out or become expensive when the GP is in town, also hotels are limited so need to be booked well in advance. The city is not very touristy, but has a few interesting things to see. Food and drink are again very cheap. An option here would be to take a flight to Poznan (very nice city) and then the train up to Bydgoszcz (2 hours). The track is a bit basic but normally produces good racing.
Torun I have been to the last 2 years, and going again this year, October can get chilly in Poland which may put some off. There is also no airport here so you would have to fly to another city and take the train or similar. Poznan, Bydgoszcz or Warsaw are reasonable options for this. The old town is very nice but largely undiscovered by foreign tourists so food and drink prices are low. Accommodation sells out on GP weekend so best to book well in advance. The stadium is purpose built for speedway and only a few years old, the track is always well prepared and produces good racing. A taxi from the city centre costs around £4-£5.
Gorzow is a bit harder to get to as train connections are a bit more limited, for example from Poznan it usually involves a change of train. Flying to eastern Germany and connecting from there may be an option but I have not ever looked in to this. In my opinion the city is not as nice as Torun and there don't seem to be many hotels in the city itself. However the stadium is only a few years old and the track produces good racing.
Tampere has its own airport, you could also fly to Helsinki and take the train (2 hours). Plenty of hotels here but everything is more expensive as it is Scandinavia. The pub right outside the railway station does beer for under €4 but most places seemed to be around the €6-€7 range. As has been well documented on this forum, the racing this year was poor, but the stadium is within walking distance of city centre hotels. The city itself is situated between 2 lakes and is a pleasant place to spend a day or 2.
I've just scrolled up and realised how much I have typed, hope this is of help to people thinking of travelling to a foreign GP. If anybody has any questions just ask, and I will do my best to help.