Hello hello hello
What are you currently collecting? I have been starting to collect modern (gasp) and just a lot of real Pokémon TCG Meta Cards (Pokémon actually has a playable game gasp) currently and want to find something to collect side by side while playing the game.
For meta-specific cards, I would recommend collecting full-art trainers. You can use the cheap normal arts in your decks and keep the expensive full-arts in your binder. Some of the special illustration rare trainers are incredible.
Something I’ve been wanting to try but never got around to is starting an art binder. I know a lot of people have something along those lines and I think it is a great cheap thing to collect.
My recommendation is to find one single card that you think has an amazing art. Then look at the artists name and start compiling cards from that artist.
I use this site to look up cards by the artist using advanced options.
https://pkmncards.com/
I think this is always a cool collection that can be relatively cheap as well.
I collect Radiants and Gold VStars 
Nice, glittery border and very affordable 
Winner stamp cards
Celebrations promos
EX era holos. Some of the most beautiful art in the hobby. Can’t go wrong.
Nah man they’re worthless
For a combination of fun to collect and relevant to the competitive scene, I would recommend gold stadiums. In particular, KATO does awesome work!
WB Kids Pokemon Card Creator Contest cards
Professor Program Promos
Staff / Nationals / Regionals etc, Promos
World Championship Promos
Victory Medals & Victory Cups
Kinda what I like 
Are there specific Pokemon that appeal to you? It’s pretty normal to collect certain Pokemon. I try to go a bit out of my way to buy Dragonite, Alakazam, Sudowoodo, and Wailmer/Wailord.
Only cards from Steam Siege set are worthless.
Anything from Gym Challenge and Gym Heroes. Those cards kick ass!
League Promos and League Challenge Promos!
League Challenge Delibird and Pokemon League Snorlax for examples;
These were set cards with a unique Cross Hatch reverse holo pattern given out at Play! Pokemon’s Pokemon League events starting in 2009 and League Challenge event starting in 2013 respectively. League Challenge promos on top of the reverse holo pattern got a stamp signifying your placement in the tournament 1st-4th as you had to win them.
Pokemon League promos were simply given out to participants. They became less interesting in the future only having the league promo stamp and the basic reverse holo effect, but League Challenge promos kept there 1st-4th placement, though losing the cross hatch pattern in 2015 with the release of BREAKthrough. They unfortunately have not been seen since Ultra Prism in the SM era.
Pokemon League promos stopped at early SWSH, but were brought back in 2022 with Play! Pokemon Prize packs. These cards have the stamp and some are holo including cosmos and a silver border in SV era packs. With the SWSH holo effect on certain cards in SWSH era packs. These promos also lost the cross hatch pattern with the release of BREAKthrough. It is interesting to note however, that for the BREAKthrough set promos they did not go with the normal reverse foil at the time instead opting for a mirror foil like patternthey have done in the past before going to the XY reverse holo pattern come BREAKpoint promos. This is present on both the Pokemon League and Pokemon Challenge Promos
Flabebe BREAKthrough Promo
Marowak BREAKthrough Promo
Chikorita BREAKpoint Promo
Greninja BREAKpoint Promo
Prize pack info
https://www.pokemon.com/us/pokemon-news/support-your-local-game-store-and-receive-play-pokemon-prize-packs
Full List of League Promos(Not including prize pack cards there is a great thread about that here)
Full List of League Challenge Promos
Wanted to challenge myself with the research here so hope you enjoy
Art Exclusives are also a cool niche. For example, these are the card arts of Gyarados that appear in English without a Japanese equivalent:
I second Radiants. For the money, they are ridiculously cool. Kids like them a lot too. They’re like modern firework-reverse holos.
Pokemon Rumble set. These are the only English cards I have and I feel they are super cool to have.
Cheers!
Some Movie Promos
Some Store Promos.
I always liked that Skuntank i dunno why. I guess because it’s basically a chunky brick. 
I want to get the card now.
I think this is aa unrated and lesser known card, but the Grey Felt Hat Pikachu Promo. Not too many people talking about it for some reason.
Jokes aside, the WOTC black star promo cards are very cool. Also agree with others that the Radiant cards are great or the modern shining cards (Shining Mew, Shining Ho-oh etc).
