Komoro Kaikoen 小諸 懐古園 is a 24-minute Shinano train しなの鉄道 ride from Karuizawa then a 3-minute walk. Exit Komoro Station 小諸, walk to the left then take the underground passage. It costs 300 yen during the day but seems to be free at night during the annual Sakura Festival (桜まつり), with lanterns tastefully lighting up the place 'til 10 pm. It took us an hour to slowly walk through the place. The last train back to Karuizawa is at 10:37 pm. There are only 1 or 2 trains per hour so time it well.
The photos online really don't do this place justice. There are more than 400 cherry blossom trees here but it's much larger and more beautiful than I thought and also much less crowded than any hanami spot in Tokyo. This year the sakura here reached full bloom 2 weeks after Tokyo and it was also 10 degrees colder in mid April. There's a pretty large area to sit under trees if you bring your own tarp and a small restaurant selling drinks 'n hot food. Make sure you climb up see the blossoms from above. It was like a dream!! Absolutely gorgeous! We were both very impressed. Definitely recommended.
Afterwards, we went on to Ueda Castle 上田城 for their night cherry blossom lightup 夜桜のライトアップ. It's 21 minutes from Komoro or 45 minutes from Shinano Karuizawa Station towards Nagano Station plus a 10-minute walk from the Oshiro Exit (walk left past the bus stops, follow the sign to the left, go down to the end of the road then make a right, and finally follow the street uphill - it's on the left). The last train back to Karuizawa is at 10:16 pm. There are only 1 or 2 trains per hour so don't miss it!
Even though the cherry blossoms at Komoro just 20 minutes away were in full bloom, the 1000 cherry blossom trees here were already leafing with only some flowers left . It was still very nice lit up at night though, with the trees' reflection in the moat. Most of the food stalls were closing/closed by 8:30 pm on a Saturday night but a few were open. The place is lit up 'til 10 pm during the Cherry Blossom Festival, which is free at night. It took us less than 1 hour to walk through.