Being you have a double crank set best option is a cassette that has a wider range you can most likely go up to a 28 on the rear, but that also depends on the rear derailleur model but I'd bet you would be safe to 28 given you have a double up front.
That said going to a triple up front would mean getting a shifter for a triple the front derailleur may work but may rub in some gear combos easer, so replacing it may be needed so the over all cost is much more.
If you fill the need for something larger than a 28 in the rear a long cage rear derailleur will be needed you can get by by adding some links to the chain you have or just replace the entire chain.