You also have the themed photography contests :

• Deranged Photography Contest by @derangedvisions
• PhotoChain challenge by @davidsimoncini
• Amazing Nature contest by @adalger

For the tags I would say that it depends on the subject of your photos, whether they are landscapes, animals, cities, cars....
you can also post on communities related to your photo subject :

If you have birds photos you can post them in Feathered Friends community run by @melinda010100 or post under the hashtag FeatheredFriends
if they are flowers you might want to post them in Always a Flower community run by @dswigle

explore the communities here and find the ones that suits you 😉

Hope that helps