What you decide to do on your 28th birthday depends on your personal tastes, and you should choose activities that you've never done so that your celebration will be memorable. Some activities don't have to be expensive. For example, attend a local food festival that has free admission. If the activity you're interested in is a little pricey, start saving up for it a few weeks before your birthday.
For those who are knowledgeable and passionate about wine, treat yourself to a wine tasting at a wine store or vineyard with your partner or a group of friends. After the wine tasting, have a picnic at a public park with snacks such as chicken-vegetable wraps, strawberry s'mores and hot soup in portable containers.
If you grew up on 1980s pop music, get tickets to a concert by an artist famous in that era. Visit a vintage clothing store and seek out items to wear to the concert that were popular in the '80s such as large and colourful headbands, bangles and brightly coloured make-up kits.
If you're on a tight budget but you want to celebrate your birthday with friends at home, you can have a potluck lunch in your backyard. Make a list of dishes you want to eat during the lunch and assign a dish for each guest to make and bring to your house. Instead of a birthday cake, purchase some candies in bulk from stores such as Sam's Club and give them out in coloured bags as party favours to the guests.