I live very close to Luton, but I never go into the town centre/shopping centre.
They (the Council, I suppose) is trying to make it nicer, but they're honestly flogging a dead horse. I never feel safe there - it has an unpleasant atmosphere.
The last time I ate in Luton was a few years ago, and we were at Mona Lisa, in the town centre, near the Hat Factory. It was a lovely little Italian place, but Ive no idea if it is still there.
Otherwise, I'd echo other posters, and suggest Dunstable Downs for nice walks/views/glider watching, Whipsnade Zoo to see the new tiny baby elephant and new baby cheetah cubs, or travel further out to St Albans (imo, nicer than Harpenden) or into London.
Enjoy the . . . What do they call them now? Gig? Concert? Show?. Whatever - have a great time.