You could treat is as a classic champagne cocktail - each glass, 1 sugar lump, small measure of brandy and top up with chilled prosecco (couple of drops angostura bitters, if you have them)
Or Bucks Fizz? 1 measure chilled fresh orange juice to 2 of prosecco, add a dash of grenadine (any supermarket)
If you have Cassis (blackcurrant), any strawberry liqueur, just a teaspoonful in the glass and top up? I really like Sourz, there's a lovely raspberry one which I use a lot - it was £7 on offer in Tesco about a week ago.
There's also a lovely melon liqueur called Midori widely available, not tried it with sparkling wine though. Also St Germain, which tastes like a cross between white grapes and lychees. It's delicious, but very expensive! The only reason I have it is because someone gave me a bottle. They keep for years though.