Can't do hotel recommendations but places you could take her:
Not just terre a terre but for daytime less fancy veggie food Brighton is fab. North Laine = mecca for it. Wai Kika Moo Kau in Kensington Gardens is yummy, extensive, all veggie. Infinity Foods Cafe does great vegan cakes and good mezze. Further out of the centre of the town, the Loving Hut on the Level is cheap and good. Wild Cherry on Queen's Park Road (further out, but if you make it that far, Queen's park - opposite it - is lovely and worth a wander) has great veggie options and is generally lovely.
Duke of York's Picturehouse near Preston Park is a lovely little cinema, first purpose-built cinema in UK and shows good stuffs. Nice bar above it too. Maybe check and see if anything interesting is coming up at The Dome or Komedia (both googlable.) Check What's On In Brighton website for any listings specific to particular days you're here.
And maybe a trip to The Royal Pavilion. Oh, and for a super-stylish cafe, Marwood Studios in the Lanes. Oh and Fabrica art gallery (converted church) also in The Lanes is free and worth a visit.