Summery Lapland - I'd say Summer in Lapland or Lapland in summer. (probably the latter)
the River Tornion–Muonionjoki in the west, the other side of which lies Sweden- the Tornion–MuonionjokiRiver in the west, on the other side of which lies Sweden
On the northern frontier on Finland,- of Finland
which is a growing travel centre- fast growing?? (I'd qualify it)
A new large national park was born- I'd be tempted to use a more conventional verb here
300 000- 300,000 (etc)
are denizens of the park- the denizens?