Since your only allod one suitcase, I don’t think you have an issue! Just pack your clothing, towels, sheets etc… all toiletries buy there, and you can also buy pillow and blankets there
Most yeshiva bochurim have the same outfit summer and winter. The only difference is that winter wardrobe includes sweaters a coat and boots or galoshen (shoe rubbers).
If you think you will be going swimming someplace include swimming trunks and if you play ball or intend to hike bring sneakers and basket ball shorts or cotton pants. For sleeping bring light pajamas or shorts…
If you are only staying the summer, dont bother taking a coat. It generally only rains and gets cold from half way through Cheshvan until Purim time.
Also, dont take seforim – not because you wont use em, but because they are cheaper there and its not worth paying extra to buy them in Chutz laaretz only to have to pay extra for baggage – and they are heavy to shlep.