Niels had a very solid, good season. I think the best thing for him to do in the off season is to play as much basketball as he can against high quality competition. I don't think his game will benefit from spending the summer in Germany. If he is able to play against some NBA guys, not just former UConn players, I think it would be invaluable. He is the type of player, who could actually compete well with professionals and it might do wonders for him. He has always had game, but he doesn't always look confident in his shot or role. Nonetheless, when he plays he usually finds a way to make a positive impact.
This may sound silly, but he might benefit from playing in one of the top Pro-Am leagues that draws some of the big time NBA talent, like The Drew League in LA. If Niels spent some time trying to guard guys like James Harden and maybe Kobe, he would get embarrassed from time to time, but it might be eye opening for him in regards to recognizing his own skill level.