First thing that I would do is give the ground a bit of TLC, but nothing too drastic. Part of the appeal of GGL is that it still has character, unlike that of somewhere like Tooting, Crawley or Kingsmeadow. I'd go about doing this by the following:
Update all the signage around the ground (getting rid of the massive exit signs) as it does look a tad dilapidated. A fresh, professional look on the exterior can do wonders for the passing public who never venture in to see a game.
A lick of paint, refurbish the seats, get rid of the ancient signage on the main stand, improve the lighting in the stands, remove the rust etc, will all go a long, long way to improving the look of the ground without losing that 'character'.
Pitch. Re-laying a new pitch isn't as expensive as you might imagine. Met Police have a cracking surface and re-lay it once every couple of years. The cost... around £30,000 at a time. Hell - even Corinthian Casuals got awarded best pitch at step 3 and 4 by the FA at the start of the season (it was in fabulous condition), and they used to have terrible problems there. It used to look more like a sandpit. Not taking away anything from the groundskeepers at GGL, but if Casuals can do it with no money, I'm sure a little investment can do wonders.
Marketing. Get the word out about Sutton Utd. Good marketing techniques can really push the attendance figures up. Give out offers, advertise in locations around the borough (the mainline station would be good), even money off coupons for attending a match in the local rag could get the gate figures up.
for example: £5 adults and children free to Sutton vs xxxx coupon.
If even 100 extra people turn up using that coupon who either are lapsed supporters (haven't been in a few years) or new altogether, and 20 of those become regulars to the matches on the back of that offer, that's £200 extra on the gate each game and roughly £4,000 per season, not withstanding the extra programme sales, food, etc. The first game may be a loss-leader (due to regulars using the coupons), but the lifetime value of a supporter is not to be underestimated!
Other initiatives should always be tested and if successful, keep 'em going.
Sutton Utd is still a name that many recognise to this day. Build on this and let everyone know you're still here and alive and kicking!
More Youth initiatives. Really get the message out there and promote what Sutton Utd does for the community. It's amazing how many parents don't realise that their local non-league teams run soccer courses, schools and academies. By offering more of this and promoting it better, this will in turn get more families through the turnstiles at the games (as their level of interest in the club will grow if their son/daughter is involved) and hopefully even a few will make the grade and eventually push through to the first team.
I'm talking door-drops, leaflet campaigns, local press and radio, PR events that gain more exposure than just the website. I know Sutton do so much in the community (like the St Helier visit before Christmas), but if I didn't frequent the website, I wouldn't know about it.
Expand the cachment area too. I don't live in Sutton or the borough (as I'm sure there are many who dont), so why not spread the word slightly more afar. Look at advertising, PR, etc into the neighbouring boroughs. It might even get other non-league clubs to step up their game (or risk losing some old/new support).
Anyway, I'm rambling now. SUFC currently do a great job now, but this is just my thoughts on how to spend the extra £££ (albeit quite modest - it's amazing how little a million goes these days!)
Oh, and I'd also buy a new Giraffe costume... ;D