How coincidental! I stayed very close to the Mantra Sierra Grand last time (the apartments look very nice, by the way!). It's a very easy walk to Oasis Mall - just head straight down Surf Parade, so, around ten minutes walk (all flat ground - not sure if this would still be difficult for your dad or not?).
You are in an excellent location (at least, I think so!), by the way! I found it so convenient because the beach was only five minutes walking distance away (if you can, I recommend getting an oceanview room), and then, you have Pacific Fair with Coles just opposite. I'm not sure if you'll be driving or not, but, if you're not, the bus stop is very close by too (opposite Mantra Bel Air, if you're heading in the direction of Surfers Paradise or the theme parks, and just outside Mantra Bel Air and the shops there, if you're wanting to head in the direction of Mermaid Beach or the Airport, etc.). I really liked the location because it was quieter than I imagine it would be on the other end closer to Oasis Mall because of all the restaurants and whatnot.
I'm not sure if you're interested in Indian food or not, but under the Mantra Sierra Grand is Saffron, which I saw was quite popular on the weekends. The Thai restaurant there (Chiangmai Thai Restaurant) was even busier, and there's also a Japanese teppanyaki restaurant there too. Unfortunately, I didn't eat any of these restaurants, but, they looked like decent options.
Not sure if you'll be looking for a cake, but, I recommend checking out Madisons Cafe' in Oasis shopping mall. We couldn't choose between the many delicious looking cakes (although, we only got a slice each) - They all looked amazing!
Good luck with your search, and I hope you all have an AMAZING trip! :)
Edited: 21 November 2011, 18:33