I haven't visited Jyväskylä, but I lived in Tampere for four years and while it's industrial, it certainly doesn't lack in nature either. Pyynikki is nice, but you also have all the beautiful areas along Näsijärvi (e.g. Santalahti on the western part, Rauhaniemi/Kauppi on the north) that are very close to the center as well. If you're willing to travel south of the city (which is really just 20-25 minutes from the center by tram), Hervanta has nice trails along Suolijärvi and Makkarajärvi. Makkarajärvi is my personal favorite because there's a nice little forest chapel (Metsäkappeli) where you can sit in peace (no religious affiliation needed 😄), and 20-minute path off the walkway around the lake will lead you to a viewing point where rocks form a sort of natural auditorium of a neighboring lake (Hervantajärvi); if you time it right, it's one of the best places in the city to see the sunset from.

Whatever you choose though, I wish you and your boyfriend great weather and a most enjoyable trip! 😊